0:00:03 > 0:00:07We think of ourselves as a global species.
0:00:12 > 0:00:17But our bodies can only survive unsupported on a fraction of the Earth's surface.
0:00:19 > 0:00:22We've evolved to live in a temperate climate at sea level,
0:00:22 > 0:00:25yet our relentless desire to explore
0:00:25 > 0:00:30has pushed us higher and deeper, to every corner of the planet.
0:00:32 > 0:00:34I'm Dr Kevin Fong.
0:00:34 > 0:00:36I study the limits the human body can endure.
0:00:36 > 0:00:40I've worked with NASA for 15 years,
0:00:40 > 0:00:43finding ways to keep people alive in orbit.
0:00:43 > 0:00:46And now, I'm going to use my own body
0:00:46 > 0:00:48to demonstrate what happens...
0:00:48 > 0:00:49Just stick with it.
0:00:49 > 0:00:54..when we go to the most extreme environments on our planet.
0:00:54 > 0:00:55I feel pretty heavy.
0:00:55 > 0:00:58Whoa!
0:00:58 > 0:00:59And that's gone.
0:00:59 > 0:01:03In this programme, I'm going down, beneath the water
0:01:03 > 0:01:06to explore our biological limits
0:01:06 > 0:01:10and to discover the technology we've had to invent
0:01:10 > 0:01:13to take us deeper and deeper.
0:01:13 > 0:01:16This is sea level, where we breathe without effort,
0:01:16 > 0:01:18without giving it a second thought.
0:01:18 > 0:01:22But three quarters of this planet are covered by a body of water
0:01:22 > 0:01:26which, for human physiology, represents only threat.
0:01:26 > 0:01:30This is the story of how our bodies respond when they are submerged into the water,
0:01:30 > 0:01:34and how technology has allowed us to explore the ocean depths.
0:01:52 > 0:01:55Water is such a hostile environment
0:01:55 > 0:01:59that we can only survive in it for as long as we can make a single breath last.
0:02:01 > 0:02:06And exactly how long that is depends on how our bodies react,
0:02:06 > 0:02:11because our physiological responses can either extend the time we can stay submerged
0:02:11 > 0:02:15or they can limit our survival time to a matter of seconds.
0:02:31 > 0:02:35This is the Royal Navy's Helicopter Underwater Escape Training facility.
0:02:35 > 0:02:39And it exists because if you're flying in helicopters over open water,
0:02:39 > 0:02:42you've got to be prepared for them to get into some difficulty.
0:02:42 > 0:02:46And so that is a simulation, a model of a Sea King helicopter.
0:02:46 > 0:02:50And today, I'm going to be inside that, upside down in the dark, trying to escape.
0:02:53 > 0:02:57All Royal Navy flight crew have to go through this training.
0:02:57 > 0:03:01It's designed to teach them the skills they need to escape from a helicopter
0:03:01 > 0:03:04after it's crashed into the open water.
0:03:04 > 0:03:10And I'm going to take part to demonstrate how one of the body's responses to sudden immersion
0:03:10 > 0:03:13can dramatically reduce our chances of survival.
0:03:14 > 0:03:16- OK, happy?- Feels good, yeah.
0:03:16 > 0:03:19Feet first, straight into the back of the module.
0:03:19 > 0:03:21- Just straight in?- Straight in.
0:03:30 > 0:03:33Head back up against the seat. That is your crash position.
0:03:33 > 0:03:36Our survival underwater
0:03:36 > 0:03:40ultimately depends on how long we can hold our breath.
0:03:40 > 0:03:44On dry land, most of us can manage about a minute.
0:03:44 > 0:03:45Break, break, break!
0:03:46 > 0:03:49But in a stressful scenario like this,
0:03:49 > 0:03:53panicking and disorientated as the helicopter hits the water,
0:03:53 > 0:03:55the situation is very different.
0:04:00 > 0:04:04My body's first response to the crash is to activate its fight or flight response.
0:04:09 > 0:04:12As the fear kicks in, the adrenal glands on top of my kidneys
0:04:12 > 0:04:15release a shot of adrenaline into my bloodstream.
0:04:17 > 0:04:20This makes my heart beat harder and faster,
0:04:20 > 0:04:23sending more blood pumping round my body,
0:04:23 > 0:04:26preparing it for a brief period of intense physical activity.
0:04:27 > 0:04:30And crucially, this exertion,
0:04:30 > 0:04:32coupled with the anxiety,
0:04:32 > 0:04:35can severely limit how long we can hold our breath.
0:04:36 > 0:04:39On dry land, the fight or flight reflex
0:04:39 > 0:04:41improves our chances of survival.
0:04:41 > 0:04:44Underwater, it does the opposite.
0:04:45 > 0:04:51In these tests, the average time most people can last is less than 20 seconds.
0:04:57 > 0:05:01So, it should be pretty straight forward.
0:05:01 > 0:05:04Open a window and...
0:05:04 > 0:05:06pull yourself out.
0:05:06 > 0:05:09In the dark, going under the water...
0:05:09 > 0:05:11slightly different prospect!
0:05:12 > 0:05:16Not sure I'd want to do that in real life!
0:05:17 > 0:05:21Escaping the helicopter is difficult enough,
0:05:21 > 0:05:25but there's a much greater threat to our survival.
0:05:26 > 0:05:28If you survive a helicopter crash at sea,
0:05:28 > 0:05:30then the real killer
0:05:30 > 0:05:33is something that the swimming pool just doesn't prepare you for.
0:05:33 > 0:05:36And that's the temperature of the water.
0:05:37 > 0:05:40The body's response to the cold is so profound
0:05:40 > 0:05:43that kills and disables even the strongest swimmers
0:05:43 > 0:05:45in a matter of seconds.
0:05:47 > 0:05:49To demonstrate this lethal reflex,
0:05:49 > 0:05:52I've come to see thermal physiologist, Mike Tipton.
0:05:54 > 0:05:56- Come and have a seat.- Excellent.
0:05:56 > 0:05:58We'll get you ready to go.
0:05:58 > 0:06:01'Mike's going to plunge me into cold water
0:06:01 > 0:06:04'to see what effect it has on my physiology,
0:06:04 > 0:06:09'and particularly how it affects my ability to hold my breath.'
0:06:09 > 0:06:12What temperature's the water at today, Mike?
0:06:12 > 0:06:13It's 12 degrees Celsius,
0:06:13 > 0:06:18which is a little bit above the average water temperature around the British Isles.
0:06:19 > 0:06:21Wired up to a heart rate monitor,
0:06:21 > 0:06:26and with a tube to measure how much air I'm breathing in and out,
0:06:26 > 0:06:29I'm ready to be dunked in the freezing tank.
0:06:29 > 0:06:31OK, Kevin, are you happy to go?
0:06:33 > 0:06:36Well, happy's a strong word, but we'll see how we go.
0:06:36 > 0:06:38OK, let's go for it.
0:06:39 > 0:06:44Before I go in, my skin temperature is a normal 37 Celsius.
0:06:47 > 0:06:49In three, two, one...
0:06:49 > 0:06:51go!
0:06:52 > 0:06:54Hold the breath, hold your breath.
0:06:54 > 0:06:58The cold shock has a massive effect on my body.
0:06:59 > 0:07:01As soon as the water touches my skin,
0:07:01 > 0:07:03it triggers a dangerous chain of reactions.
0:07:06 > 0:07:08My pulse shoots up to over twice its normal rate.
0:07:11 > 0:07:15My muscles go into spasm in an attempt to generate more heat,
0:07:15 > 0:07:18causing my body to shiver uncontrollably.
0:07:19 > 0:07:22And although I'm trying to hold my breath,
0:07:22 > 0:07:23I just can't.
0:07:23 > 0:07:26Now breathe freely, breathe freely.
0:07:27 > 0:07:28Looking good.
0:07:29 > 0:07:31Just stick with it.
0:07:33 > 0:07:35That's good. Starting to settle down.
0:07:37 > 0:07:39That's looking fine.
0:07:39 > 0:07:41Take the mouth piece out and have a chat.
0:07:41 > 0:07:44You can take your nose clip off as well.
0:07:44 > 0:07:45So...
0:07:46 > 0:07:47..subjectively, how was that?
0:07:47 > 0:07:49Very painful.
0:07:49 > 0:07:52Couldn't hold my breath there at all.
0:07:52 > 0:07:54Really, really trying,
0:07:54 > 0:07:58and I can hold my breath for well over a minute at rest.
0:07:59 > 0:08:02Quite shocking, actually. Thought I'd be able to do it.
0:08:02 > 0:08:06Really motivated to try and hold my breath there.
0:08:08 > 0:08:10After just a couple of minutes in the water,
0:08:10 > 0:08:13my skin temperature has plummeted.
0:08:13 > 0:08:16But it's the data Mike has collected
0:08:16 > 0:08:18that reveals just how lethal the cold shock can be.
0:08:20 > 0:08:24We know that when you were completely at rest, when we first sat you in the chair,
0:08:24 > 0:08:26you had a heart rate of about 50 beats per minute.
0:08:26 > 0:08:30And that's gone up to just over 100, just before you went in.
0:08:30 > 0:08:36So that's being driven by very understandable anxiety.
0:08:36 > 0:08:39During that first 20 seconds of immersion,
0:08:39 > 0:08:43- it was up to around about 145 beats per minute.- Wow!
0:08:43 > 0:08:47That's a remarkably high heart rate for somebody who's essentially sitting still.
0:08:47 > 0:08:52But, of course, that's all being driven by those cold receptors that are making your heart beat faster.
0:08:54 > 0:08:58It's clear that my pulse is affected by the cold water,
0:08:58 > 0:09:01but most shocking of all was my respiration rate
0:09:01 > 0:09:03as I'm trying to hold my breath.
0:09:05 > 0:09:09There's where you start breath-holding, where you went into the water.
0:09:09 > 0:09:14Now we've got the data and we've analysed it, we can see your breath hold time is 12 seconds.
0:09:14 > 0:09:19That's still about twice what we would expect somebody wearing the amount of your clothing
0:09:19 > 0:09:22to achieve on average, so it's a good performance.
0:09:22 > 0:09:24Is that true, that the average person,
0:09:24 > 0:09:27wearing no real protective clothing,
0:09:27 > 0:09:29holds their breath six seconds?
0:09:29 > 0:09:31Five or six seconds is average.
0:09:35 > 0:09:38To illustrate how serious this is,
0:09:38 > 0:09:41you just need to apply that six-second breath-hold
0:09:41 > 0:09:44to a situation like the helicopter crash simulator.
0:09:47 > 0:09:51The average person would stand no chance of getting out of the helicopter.
0:09:51 > 0:09:54I lasted for another six seconds,
0:09:54 > 0:09:56but it's still not enough.
0:09:56 > 0:09:59I would still have been underwater
0:09:59 > 0:10:02when that uncontrollable urge to take a breath came.
0:10:03 > 0:10:05And if you inhale underwater,
0:10:05 > 0:10:08the consequences are disastrous.
0:10:09 > 0:10:12So now you're breathing uncontrollably,
0:10:12 > 0:10:17and you're shifting three, four, five litres in and out of the lung every breath.
0:10:17 > 0:10:20And just one-and-a-half of those litres has to be water rather than air
0:10:20 > 0:10:23for you to have crossed the lethal dose for drowning.
0:10:23 > 0:10:26It's pretty terrifying data to look at
0:10:26 > 0:10:29cos this is my 12 second measly 12 second - breath-hold,
0:10:29 > 0:10:32and then I take a nearly two-litre breath.
0:10:32 > 0:10:34So every single one of these breaths
0:10:34 > 0:10:37fills my lungs with a lethal volume of water.
0:10:37 > 0:10:39Every single one these breaths!
0:10:39 > 0:10:41That kills me. That kills me. That kills me.
0:10:41 > 0:10:44Every one of these breaths that I can't control
0:10:44 > 0:10:46drowns and kills me.
0:11:03 > 0:11:09The cold shock response accounts for something like 60% of all drownings in British waters each year.
0:11:09 > 0:11:13It kills strong swimmers, even in calm conditions.
0:11:13 > 0:11:18And it's a shocking reminder of just how poorly adapted to water we really are.
0:11:24 > 0:11:27Given how utterly hostile water is to our bodies,
0:11:27 > 0:11:31you'd think that we'd just try and steer clear of it altogether.
0:11:31 > 0:11:33But we don't. We're strangely drawn to it
0:11:33 > 0:11:36and the ocean depths, and we try to explore.
0:11:36 > 0:11:41When we do, we find surprising aspects of our physiology and biology
0:11:41 > 0:11:45that are better adapted to the life aquatic than we think.
0:12:07 > 0:12:12Sara Campbell is a prime example of just how well our bodies can adapt to the water.
0:12:14 > 0:12:17She's able to stay underwater for longer
0:12:17 > 0:12:20and dive much deeper than almost any other human.
0:12:24 > 0:12:26It feels like a very natural place for me to be.
0:12:26 > 0:12:29For as long as I can remember, even before I learnt how to swim,
0:12:29 > 0:12:31I loved being underwater.
0:12:34 > 0:12:39I've often questioned where this talent has come from, this ability,
0:12:39 > 0:12:41and I just have to remember that I'm a mammal
0:12:41 > 0:12:45and we all evolved from the seas millions of years ago.
0:12:46 > 0:12:48And for me, it feels like home.
0:12:50 > 0:12:53Sara has trained her natural ability
0:12:53 > 0:12:56to become a world champion free-diver.
0:12:56 > 0:13:00She's able to harness a range of physiological responses
0:13:00 > 0:13:03that allow her to survive underwater for extended periods.
0:13:05 > 0:13:08And she's promised to teach me the first step,
0:13:08 > 0:13:10which is how to hold my breath for longer.
0:13:13 > 0:13:17So the first thing that we start with is teaching how to breathe correctly.
0:13:17 > 0:13:22Most people have a very shallow breath and they're not maximising the capacity of their lungs.
0:13:22 > 0:13:25So what we need to think about is a three part breath.
0:13:25 > 0:13:26Starting with the belly,
0:13:26 > 0:13:28moving up to the chest
0:13:28 > 0:13:30and finishing by topping up into the shoulders.
0:13:33 > 0:13:37We usually use only 10% of our lung capacity,
0:13:37 > 0:13:40but Sara's technique makes use of their full volume,
0:13:40 > 0:13:43preparing her body for a breath-hold attempt.
0:14:01 > 0:14:03All right?
0:14:03 > 0:14:05Do I need the sound effects as well?
0:14:06 > 0:14:09- Try doing it as if you're sucking through a straw.- OK.- Yeah?
0:14:21 > 0:14:25Yeah. OK, so for the next part, we're going to do a breath-hold.
0:14:25 > 0:14:29So we're going to lie you down, do some long, deep breathing, do some relaxation.
0:14:29 > 0:14:30All right.
0:14:32 > 0:14:35So the most important thing now is for you to relax.
0:14:35 > 0:14:37And be nice and calm.
0:14:37 > 0:14:39So when you feel ready,
0:14:39 > 0:14:41hold your breath.
0:14:41 > 0:14:46Give me the signal with a finger that you've begun your breath-hold.
0:14:48 > 0:14:50With every breath,
0:14:50 > 0:14:53we're inhaling oxygen and exhaling carbon dioxide.
0:14:56 > 0:14:57But when we hold our breath,
0:14:57 > 0:15:01that gas exchange in our lungs grinds to a halt
0:15:01 > 0:15:03and changes our physiology.
0:15:04 > 0:15:07The oxygen levels in our blood start falling,
0:15:07 > 0:15:10but more importantly, the carbon dioxide levels rise.
0:15:12 > 0:15:16And it's this rise in CO2 that's detected by receptors in our brain,
0:15:16 > 0:15:19which in turn trigger our urge to breathe.
0:15:21 > 0:15:24And one of the first places to feel those signals is the diaphragm,
0:15:24 > 0:15:30which starts to contract involuntarily in an attempt to inflate the lungs.
0:15:32 > 0:15:36What I have to do is try and ignore those spasms.
0:15:36 > 0:15:39Though, of course, you should never try to do this at home.
0:15:40 > 0:15:45Here we can see Kevin's diaphragm pulling down...
0:15:46 > 0:15:47..wanting to breathe.
0:15:48 > 0:15:50That's good. Keep calm, Kevin.
0:15:51 > 0:15:53Keep relaxed.
0:15:53 > 0:15:56When you feel that urge to breathe, it's not coming from a lack of oxygen,
0:15:56 > 0:16:01it's coming from the fact that the carbon dioxide levels in your blood are increasing.
0:16:01 > 0:16:04But actually, you have more than enough oxygen in your body
0:16:04 > 0:16:07to continue for 30 seconds, one minute, two minutes more.
0:16:10 > 0:16:12Three, two, one...
0:16:12 > 0:16:13and breathe. Well done.
0:16:16 > 0:16:17Good. How do you feel?
0:16:17 > 0:16:19Out of breath!
0:16:19 > 0:16:22So you've got your contractions at one minute, 30.
0:16:22 > 0:16:26And you held to a one minute, 50. So it's a very good start.
0:16:28 > 0:16:31Not quite enough for championship free-diving, I don't think.
0:16:31 > 0:16:33Not yet! Well done.
0:16:33 > 0:16:35What can you manage?
0:16:35 > 0:16:38- Oh, I've done 5.12.- 5.12!
0:16:38 > 0:16:40But if you compare that to the world records,
0:16:40 > 0:16:43the women are at 8 minutes, 20
0:16:43 > 0:16:45and the men at 11.35.
0:16:45 > 0:16:46- 11.35?!- Mmm.
0:16:47 > 0:16:50- Little bit of practice. - I'm sure that can't be good for you!
0:16:50 > 0:16:56But you see how the body is able to adapt and deal with extreme lacks of oxygen
0:16:56 > 0:16:58and still survive. It's really incredible.
0:17:09 > 0:17:14From her base here in Egypt, Sara has trained her remarkable ability
0:17:14 > 0:17:16to become one of the world's best free-divers.
0:17:18 > 0:17:21A sport in which the aim is to dive as deep as possible
0:17:21 > 0:17:22on a single breath.
0:17:25 > 0:17:30It marks the limit of how far we can explore underwater without the aid of technology,
0:17:30 > 0:17:34and it pushes human physiology to the very limit.
0:18:01 > 0:18:04So Sara's just here,
0:18:04 > 0:18:06looking very comfortable at 10 metres,
0:18:06 > 0:18:10and this, for her, is just the beginning.
0:18:10 > 0:18:14She can manage to get down another 90 metres to almost 100 metres.
0:18:16 > 0:18:19And to do that, her body has to undergo
0:18:19 > 0:18:21some quite impressive physiological changes.
0:18:24 > 0:18:25The first change happens
0:18:25 > 0:18:28as soon as the water touches the nerve endings in her face.
0:18:31 > 0:18:34In response to the water, signals are sent to the heart,
0:18:34 > 0:18:36telling it to slow down,
0:18:36 > 0:18:40reducing its work rate and conserving oxygen to be used by the brain.
0:18:47 > 0:18:48Let's have a look.
0:18:51 > 0:18:53Now that's nice and slow.
0:18:56 > 0:18:59It's probably getting on for...
0:18:59 > 0:19:01less than 50 beats per minute there.
0:19:03 > 0:19:05And that's pretty impressive.
0:19:05 > 0:19:09When Sara gets down, closer to 100 metres...
0:19:10 > 0:19:14..her heart rate can get down as low as 36 beats per minute.
0:19:17 > 0:19:21As she dives deeper, her physiology changes even more.
0:19:23 > 0:19:25Her circulation changes.
0:19:26 > 0:19:29The blood vessels in her limbs constrict,
0:19:29 > 0:19:31reducing blood flow to the body's periphery,
0:19:31 > 0:19:34further conserving her oxygen supplies.
0:19:37 > 0:19:39At depths below 50 metres,
0:19:39 > 0:19:42the pressure can cause fluid to leak into her lungs.
0:19:42 > 0:19:45It's a potentially dangerous response,
0:19:45 > 0:19:49but it may protect the lungs from further damage on very deep dives.
0:19:56 > 0:20:02It's these changes that have allowed Sara to dive straight down for over a minute-and-a-half
0:20:02 > 0:20:04to the incredible depth of 96 metres.
0:20:09 > 0:20:12This ability to change our physiology underwater
0:20:12 > 0:20:15is something we share with all mammals.
0:20:15 > 0:20:17It's called the dive reflex,
0:20:17 > 0:20:20and it's a throwback to our distant evolutionary past
0:20:20 > 0:20:23when we were all aquatic creatures.
0:20:32 > 0:20:35Today, the dive reflex is strongest in marine mammals
0:20:35 > 0:20:37which are able to survive underwater
0:20:37 > 0:20:41because they can reduce their demand for oxygen to almost nothing.
0:20:44 > 0:20:47While Sara can dive to nearly 100 metres,
0:20:47 > 0:20:50a sperm whale can reach 3,000 metres,
0:20:50 > 0:20:53and stay submerged for 90 minutes.
0:21:00 > 0:21:04What you have to realise when you're watching Sara do this incredible thing,
0:21:04 > 0:21:06diving free on a single breath,
0:21:06 > 0:21:10is you forget that this is a limited lease for her.
0:21:10 > 0:21:17She's fighting the urge to breathe as the carbon dioxide builds up in her body, in her bloodstream,
0:21:17 > 0:21:20and at the same time, the oxygen levels are falling.
0:21:20 > 0:21:23So sooner or later, something has to happen.
0:21:23 > 0:21:26Either she has to return to the surface to take a breath,
0:21:26 > 0:21:27or she's going to pass out.
0:21:30 > 0:21:35And the danger of passing out gets greater the closer she gets to the surface.
0:21:39 > 0:21:41Already oxygen-starved,
0:21:41 > 0:21:45the sudden drop in pressure further reduces the level of oxygen in her bloodstream.
0:21:47 > 0:21:49Which can have devastating consequences.
0:22:01 > 0:22:04This is Sara diving in the Bahamas,
0:22:04 > 0:22:07attempting to become the first woman to dive to 100 metres
0:22:07 > 0:22:10on a single breath.
0:22:10 > 0:22:12Having reached her target,
0:22:12 > 0:22:14all she had to do to claim the record
0:22:14 > 0:22:16was return safely to the surface.
0:22:22 > 0:22:27Hold yourself up. Breathe, breathe. Nose clip. Nose clip...
0:22:27 > 0:22:28- Grab her!- Sara, Sara, Sara?
0:22:28 > 0:22:30Get the weight off.
0:22:31 > 0:22:33Blow in her face.
0:22:35 > 0:22:38Come on, Sara. Come on, Sara.
0:22:38 > 0:22:39Breathe.
0:22:40 > 0:22:41Breathe.
0:22:42 > 0:22:43Sara, all right.
0:22:43 > 0:22:46Without the safety divers on hand,
0:22:46 > 0:22:48- Sara could have drowned.- I'm OK.
0:22:50 > 0:22:54As it was, because she passed out, her record didn't stand.
0:22:54 > 0:22:58Blackouts like these are common amongst free-divers,
0:22:58 > 0:23:01and they reveal our underlying frailty in the water.
0:23:08 > 0:23:11Sara is a world class free-diver,
0:23:11 > 0:23:13and despite all of her training,
0:23:13 > 0:23:15and all the precautions that she takes,
0:23:15 > 0:23:18she cannot make this activity completely safe.
0:23:19 > 0:23:24She's at the very, very edge of her physiology each time she descends into the deep.
0:23:24 > 0:23:28And it's a sobering reminder, reading the text books and the case histories
0:23:28 > 0:23:31of people who've had horrific accidents, sometimes fatal accidents,
0:23:31 > 0:23:34undertaking exactly this type of feat.
0:23:35 > 0:23:39And so our stays underneath the surface of the ocean, unsupported,
0:23:39 > 0:23:42are limited to a few hundred seconds, a few tens of metres.
0:23:42 > 0:23:45And if we want to stay longer and go deeper,
0:23:45 > 0:23:49we're going to need something in the way of technology that can protect us.
0:23:51 > 0:23:54We've been inventing technology to take us deeper,
0:23:54 > 0:23:57and stay longer underwater, for hundreds of years.
0:23:59 > 0:24:01The first devices were diving bells
0:24:01 > 0:24:05in which people could descend beneath the waves
0:24:05 > 0:24:06in a trapped pocket of air.
0:24:09 > 0:24:12Legend has it that as long ago as 300BC,
0:24:12 > 0:24:16Alexander the Great was using a giant glass diving bell.
0:24:16 > 0:24:20But it wasn't until the 18th century that our ability to explore the depths
0:24:20 > 0:24:25was revolutionized by the invention of a diving suit
0:24:25 > 0:24:28that gave us the freedom to move around underwater.
0:24:41 > 0:24:44'To experience what early diving was like,
0:24:44 > 0:24:47'I've come to this ultra-modern aquarium in Cheshire
0:24:47 > 0:24:51'to meet Howard Dykes from the Historical Diving Society.'
0:24:51 > 0:24:52This is your kit?
0:24:52 > 0:24:55'He's going to let me try out his antique diving suit.'
0:24:55 > 0:25:00That's an Admiralty pattern 6-bolt standard dress diving helmet.
0:25:00 > 0:25:03Until what time were they using all of this?
0:25:03 > 0:25:06- Late '60s, early '70s. - As late as that?- Yeah.
0:25:06 > 0:25:10- They still dive on kit like this in other countries.- Gosh!
0:25:10 > 0:25:11All this is fantastic.
0:25:11 > 0:25:15It's one of these things that you have memories of it from childhood,
0:25:15 > 0:25:18reading comic books or watching old black and white films.
0:25:18 > 0:25:20It's quite incredible to see one up close.
0:25:20 > 0:25:22That's why I've started diving.
0:25:22 > 0:25:25- Really?- Yeah, with my Action Man.
0:25:25 > 0:25:28- I had my Action Man...- I remember, yeah!- ..the standard dress diver.
0:25:28 > 0:25:32- And this is why I got into diving. - Yeah, I think it was Tintin for me.
0:25:32 > 0:25:33That's it, Red Rackham's Treasure.
0:25:33 > 0:25:36Red Rackham's Treasure, that's the one!
0:25:36 > 0:25:39Brilliant. I can't believe I'm going to get the chance to put one on!
0:25:42 > 0:25:45There's more to the suit than just the helmet.
0:25:45 > 0:25:48It comes with a waterproof canvas suit,
0:25:48 > 0:25:50a heavy-duty brass collar,
0:25:50 > 0:25:52and, of course, matching shoes.
0:25:53 > 0:25:56They come in a right and a left, these?
0:25:56 > 0:25:59- Yeah, with the buckles on the outside.- Ah, OK.
0:25:59 > 0:26:03And we're all happy that this is all completely safe?
0:26:03 > 0:26:04- No.- No?
0:26:04 > 0:26:05OK, great! Brilliant(!)
0:26:05 > 0:26:08Well, let's get on then!
0:26:08 > 0:26:09Right now I am...
0:26:10 > 0:26:11..wearing a...
0:26:13 > 0:26:15..very heavy metal collar,
0:26:15 > 0:26:16some lead-weighted shoes,
0:26:16 > 0:26:19next to a body of water full of sharks.
0:26:19 > 0:26:20What could go wrong(?)
0:26:24 > 0:26:26- 'Hello, diver?'- Hello.
0:26:26 > 0:26:28'Can you hear me OK?'
0:26:28 > 0:26:30I can hear you fine, thank you.
0:26:31 > 0:26:32And here I go.
0:26:33 > 0:26:34First step.
0:26:34 > 0:26:37That's really quite heavy.
0:26:38 > 0:26:39I am underwater!
0:26:42 > 0:26:44See if I can make a step forwards.
0:26:46 > 0:26:48That all feels pretty good.
0:26:48 > 0:26:49That's great!
0:26:51 > 0:26:52And I am diving.
0:26:55 > 0:26:59That's really fantastic, actually.
0:26:59 > 0:27:01Some of those lovely sharks over there
0:27:01 > 0:27:06wondering what the hell it is that this crazy guy is doing.
0:27:09 > 0:27:13'In these suits, divers could stay underwater for many hours,
0:27:13 > 0:27:17'reaching depths of well over 100m,
0:27:17 > 0:27:21'but they were totally dependent on hand-operated pumps on the surface.'
0:27:25 > 0:27:29Despite the fact that I'm in a few metres of water,
0:27:29 > 0:27:35surrounded by sharks, the thing that most worries me is not them,
0:27:35 > 0:27:39it's that that pump being operated by a couple of guys on the surface,
0:27:39 > 0:27:43running air down the hose, that's entering my helmet,
0:27:43 > 0:27:46that all of that simple but all-important life support
0:27:46 > 0:27:48continues to work.
0:27:52 > 0:27:56'The deeper I dive, the harder they have to pump
0:27:56 > 0:27:59'to maintain the air pressure inside the suit.
0:27:59 > 0:28:04'If they stop, I'd soon feel the pressure of the water outside.'
0:28:05 > 0:28:08There's an eel. I'll try not to tread on him.
0:28:09 > 0:28:13'At greater depths, that crush can be fatal. In the worst cases,
0:28:13 > 0:28:17'where the valves in the helmet failed catastrophically,
0:28:17 > 0:28:21'the divers' whole bodies were squashed into their helmets.
0:28:21 > 0:28:25'A fate delicately known as "the squeeze".'
0:28:29 > 0:28:32You come to realise that the whole endeavour of diving
0:28:32 > 0:28:36wasn't invented so that someone could have a bit of fun underwater,
0:28:36 > 0:28:40it was invented so that we could take ourselves
0:28:40 > 0:28:43to this extreme environment...
0:28:44 > 0:28:49..and not just visit, but work here.
0:28:51 > 0:28:54'It was divers in suits just like this
0:28:54 > 0:28:58'who built many of the great Victorian engineering projects -
0:28:58 > 0:29:01'the bridges and tunnels that are still used today.'
0:29:04 > 0:29:07- RADIO:- 'It's time to come home now.'
0:29:09 > 0:29:12Apparently, it's time to go home.
0:29:23 > 0:29:26- RADIO:- 'Can the guys stop pumping now?'
0:29:26 > 0:29:28Yes, porthole's open.
0:29:28 > 0:29:31Thank you, guys. Thanks so much for keeping me alive.
0:29:34 > 0:29:38It was brilliant. Really, really fantastic experience.
0:29:38 > 0:29:42I was a bit disappointed I didn't get to wander around a bit more.
0:29:42 > 0:29:45Very disappointed to come up so soon.
0:29:45 > 0:29:48I'll have to come back and do it again another day.
0:29:55 > 0:29:59This very simple but very robust piece of equipment
0:29:59 > 0:30:01opened up a whole new world for us -
0:30:01 > 0:30:05allowed us to do things underwater we'd never been able to do before.
0:30:05 > 0:30:08But with all of that advantage came a drawback
0:30:08 > 0:30:10and that was an industrial disease,
0:30:10 > 0:30:15a disease that would injure people, paralyse them, kill some of them,
0:30:15 > 0:30:17and that was the bends.
0:30:23 > 0:30:27'The bends is still every diver's worst nightmare.
0:30:27 > 0:30:30'To demonstrate what causes it,
0:30:30 > 0:30:34'I've come to the Diving Diseases Research Centre in Plymouth.'
0:30:37 > 0:30:41'I'm about to go on one of the deepest dives of my life
0:30:41 > 0:30:44'but I'm not even going to get wet.
0:30:44 > 0:30:47'I'm going diving in a pressure chamber
0:30:47 > 0:30:51'which can replicate the conditions of being deep underwater.'
0:30:52 > 0:30:56So this is the chamber we're going to be diving in today.
0:30:56 > 0:31:00They're going to fill it with compressed air
0:31:00 > 0:31:03and get it down to 40m. By the time we're down there,
0:31:03 > 0:31:07the pressure in here is going to be five times what it would be at the surface
0:31:07 > 0:31:10and, although it's going to look the same, to my body,
0:31:10 > 0:31:14my biology, my physiology, it's going to be a whole new world.
0:31:14 > 0:31:19- Good to go. - Understood. That'll be a minute 25, and we're leaving.
0:31:22 > 0:31:27'To simulate depth, air is pumped into the chamber to increase the pressure.
0:31:27 > 0:31:30'When there's twice as much air inside,
0:31:30 > 0:31:35'the pressure has doubled and it's equivalent to being 10m underwater.
0:31:35 > 0:31:40'As more air comes in, you can even see the effects.
0:31:40 > 0:31:43'Anything with an air pocket in it is squashed.
0:31:43 > 0:31:46'If I didn't clear my ears constantly,
0:31:46 > 0:31:48'my ear drums would burst.
0:31:48 > 0:31:52'It's even too much for some of our cameras.
0:31:52 > 0:31:56'By the time five times more air has been pumped into the chamber,
0:31:56 > 0:32:01'we're at the same pressure as we would be 40m underwater.
0:32:01 > 0:32:07'Every bit of my body is now being squeezed five times harder than it would be at the surface
0:32:07 > 0:32:11'and that squeeze is affecting me in some unexpected ways.'
0:32:12 > 0:32:15I think I've developed a lisp.
0:32:16 > 0:32:19What's that about? I don't like that.
0:32:19 > 0:32:23Seriously, why have I got a lisp?
0:32:23 > 0:32:27The reason my voice sounds quite so strange, erm...
0:32:27 > 0:32:33is because the air is sufficiently dense at this depth
0:32:33 > 0:32:38that the speed of sound is faster, the pitch is higher.
0:32:38 > 0:32:43And not just that, but two out of three of our cameras have given up,
0:32:43 > 0:32:47stopped working, so this is going to be a challenging bit of television.
0:32:50 > 0:32:55We're now going to sit here for the next half an hour or so
0:32:55 > 0:32:58and hope that our bodies respend...
0:32:58 > 0:33:02our bodies, we are going to hope that our bodies respond better
0:33:02 > 0:33:04than the cameras do.
0:33:04 > 0:33:08'In addition to my squeaky voice, you may have noticed
0:33:08 > 0:33:12'that I'm having trouble speaking or even thinking clearly.
0:33:12 > 0:33:17'It's because just breathing air at this pressure makes us drunk.
0:33:17 > 0:33:21'It's an effect best demonstrated with a bottle of champagne.'
0:33:21 > 0:33:26So this is a bottle of champagne, normally very fizzy
0:33:26 > 0:33:30and the sort of thing you could have a good celebration with.
0:33:30 > 0:33:32Let's just take the cork off this.
0:33:33 > 0:33:39And there you go. Very impressively unbubbly bottle of bubbly.
0:33:42 > 0:33:44The reason that's happening
0:33:44 > 0:33:49and the reason this looks like a very unappetising and flat...
0:33:49 > 0:33:53Look at that - a flat glass of champagne.
0:33:55 > 0:33:59..is because the pressure is keeping the carbon dioxide
0:33:59 > 0:34:02firmly in solution, so...
0:34:03 > 0:34:05..this doesn't want to fizz.
0:34:06 > 0:34:12'The reason the champagne is flat is that the higher pressure is stopping the bubbles forming
0:34:12 > 0:34:17'by forcing the gas to stay dissolved in the liquid.'
0:34:18 > 0:34:21Exactly the same thing is happening in my body.
0:34:21 > 0:34:27In my lungs, the pressure is forcing the gases in the air to dissolve in my blood.
0:34:28 > 0:34:31There's now many times more gas dissolved in my body
0:34:31 > 0:34:35than there was at the surface and most of it is nitrogen.
0:34:38 > 0:34:43That dissolved nitrogen affects my brain like a powerful narcotic.
0:34:43 > 0:34:46It's what divers call "the narcs".
0:34:49 > 0:34:53You don't need champagne to get drunk when you're at this depth.
0:34:53 > 0:34:56The nitrogen in your body is enough to start to get you drunk.
0:34:56 > 0:34:59Nitrogen, which is normally inert at sea level,
0:34:59 > 0:35:03begins to behave like an anaesthetic agent at this depth -
0:35:03 > 0:35:06starts to make you feel giddy, light-headed,
0:35:06 > 0:35:10starts to make you behave slightly abnormally.
0:35:10 > 0:35:15And, erm...so, it's just as well, because the alcohol tastes rubbish.
0:35:15 > 0:35:20'While the narcs are a lot of fun in the controlled environment of the chamber,
0:35:20 > 0:35:23'they can be very dangerous underwater.
0:35:23 > 0:35:27'The only way to relieve the symptoms is to come to a shallower depth.
0:35:29 > 0:35:33'But it's on the return to the surface that the danger of the bends begins.
0:35:33 > 0:35:35'As the pressure is reduced,
0:35:35 > 0:35:39'the dissolved gas begins to come out of solution again.
0:35:39 > 0:35:42'It's vitally important that we come up slowly
0:35:42 > 0:35:47'to allow the extra nitrogen in my body to be released gradually through the lungs.
0:35:47 > 0:35:50'If we were to come up too quickly
0:35:50 > 0:35:54'the gas would suddenly come out of solution, forming bubbles
0:35:54 > 0:35:57'in my tissues and blood vessels like bubbles in champagne.
0:35:58 > 0:36:01'It's those bubbles that cause the bends
0:36:01 > 0:36:04'and they can have devastating consequences.'
0:36:23 > 0:36:27These are the worst consequences of an attack of the bends.
0:36:27 > 0:36:31This is what happens when the bubbles that evolve in your tissues and blood stream get in
0:36:31 > 0:36:34and block vessels either by passing through and causing damage
0:36:34 > 0:36:36or lodging in those vessels.
0:36:36 > 0:36:39These two pictures are people with brain damage.
0:36:39 > 0:36:42The white spots you can see here are bits of dead brain -
0:36:42 > 0:36:46brain that's died because it has lost its blood supply.
0:36:46 > 0:36:51Down here you're seeing someone who's had bubbles evolve in their bone marrow
0:36:51 > 0:36:55and all those black spots you can see there are areas of expanded gas
0:36:55 > 0:36:58that are going to cause a lot of pain and problem later on.
0:36:58 > 0:37:01Most alarmingly, this up here,
0:37:01 > 0:37:03a shot of brain in a post mortem,
0:37:03 > 0:37:06and you can actually see bubbles here
0:37:06 > 0:37:08expanded in the veins around the brain
0:37:08 > 0:37:13that have caused a blockage and contributed to this patient's death.
0:37:13 > 0:37:16These are lethal or potentially lethal injuries.
0:37:20 > 0:37:23'With the risks of drowning and the bends,
0:37:23 > 0:37:26'it makes you wonder why anyone would want go diving.
0:37:26 > 0:37:29'But our inherent desire to keep pushing the boundaries
0:37:29 > 0:37:32'means millions and millions of people around the world
0:37:32 > 0:37:35'now count diving as a hobby.'
0:37:35 > 0:37:40'The reason we can do that is all down to one remarkable invention.'
0:37:46 > 0:37:49It was this type of equipment, just over half a century ago,
0:37:49 > 0:37:52that revolutionised the underwater experience.
0:37:52 > 0:37:56We take them for granted now because we see them everywhere
0:37:56 > 0:37:59but this bit of kit has two very important components.
0:37:59 > 0:38:04A tank of compressed air so you could take your air supply with you and not depend on the surface,
0:38:04 > 0:38:08and then, secondly, this pressure-reducing demand valve.
0:38:08 > 0:38:12That brought this very high pressure down to something you could breathe
0:38:12 > 0:38:15but, also, it allowed you to draw air off on demand
0:38:15 > 0:38:19and allowed this finite supply of air to last much longer.
0:38:34 > 0:38:40'It was pioneered by French marine biologist, Jacques-Yves Cousteau.
0:38:40 > 0:38:46'Carrying our own air supply gave everyone the freedom of the ocean.
0:38:46 > 0:38:50'It allowed us to dive independently for more than an hour at a time
0:38:50 > 0:38:54'and reach depths of 30m and beyond in relative safety.
0:39:04 > 0:39:09'Since Cousteau's time, scuba technology has come a long way.
0:39:09 > 0:39:14'Modern divers carry much more equipment and gadgetry
0:39:14 > 0:39:18'but the basic principle has remained unchanged.'
0:39:29 > 0:39:3230 metres. Very, very, very beautiful.
0:39:35 > 0:39:38All of this remarkable engineering,
0:39:38 > 0:39:42this self-contained underwater breathing apparatus,
0:39:42 > 0:39:46allows us to explore underwater freely.
0:39:46 > 0:39:51But it only just gives us access to the first few tens of metres.
0:39:51 > 0:39:57Out there is an entire ocean that I can't get near dressed like this.
0:39:57 > 0:40:03For that, you need much more substantial equipment.
0:40:08 > 0:40:10With basic scuba gear,
0:40:10 > 0:40:14we're limited to diving a few tens of metres below the surface.
0:40:14 > 0:40:18But we want and need to go much deeper than that.
0:40:22 > 0:40:24This is the North Sea.
0:40:24 > 0:40:27Its floor is home to some of the world's richest deposits
0:40:27 > 0:40:29of oil and minerals,
0:40:29 > 0:40:32but to get to those deposits we have to put men on the sea floor,
0:40:32 > 0:40:37often over 100 metres below the surface.
0:40:37 > 0:40:42And doing that is a massive technical and physiological challenge.
0:40:50 > 0:40:55This is the Toisa Polaris, a very impressive-looking dive support vessel.
0:40:55 > 0:41:00This vast ship complete with helicopter pad, cranes,
0:41:00 > 0:41:05100 crew, it's all here to support just a handful of men
0:41:05 > 0:41:07diving in the North Sea.
0:41:08 > 0:41:13To reach the bottom of the sea, the divers take a diving bell.
0:41:13 > 0:41:18An air-filled capsule pressurised to up to 35 times atmospheric pressure.
0:41:22 > 0:41:24Chad, what is this place?
0:41:24 > 0:41:29This here is our dive bell hanger and this here is the dive bell.
0:41:30 > 0:41:33So how does that get into the ocean?
0:41:33 > 0:41:36We trolley it over, you can see the trolley tracks.
0:41:36 > 0:41:38Oh, right, OK.
0:41:38 > 0:41:42Then we come over here to the moon pool and then it descends down.
0:41:42 > 0:41:44So that's kind of like your taxi to work?
0:41:44 > 0:41:46Pretty much, pretty much.
0:41:47 > 0:41:53The diving bells provide a vital lifeline to the divers in the water.
0:41:53 > 0:41:56An umbilical cord which connects the ship to the bell
0:41:56 > 0:41:58and the bell to the divers' suits.
0:41:59 > 0:42:04It supplies hot water for warmth, air to breathe, power for light
0:42:04 > 0:42:06and communications to the surface.
0:42:07 > 0:42:10- Can we have a look inside? - Definitely.
0:42:11 > 0:42:14How long does it take you to reach the bottom
0:42:14 > 0:42:16once you come off the ship
0:42:16 > 0:42:19and you're going down in one of these?
0:42:19 > 0:42:23Well, there's a few variables, depending how deep we're diving
0:42:23 > 0:42:24and how bad the weather is
0:42:24 > 0:42:28but usually it'll take from 20 to 30 minutes.
0:42:28 > 0:42:30Wow!
0:42:30 > 0:42:34It's a crazy-looking place. Really cramped in there,
0:42:34 > 0:42:38you can't imagine that three people occupy it.
0:42:38 > 0:42:42One of those poor blokes has to stay in for the whole six-hour dive.
0:42:44 > 0:42:48Once at the bottom, the divers leave the bell wearing a diving helmet
0:42:48 > 0:42:52strong enough to withstand the immense pressures.
0:42:53 > 0:42:56But impressive as all this hardware is,
0:42:56 > 0:43:00the really astonishing aspects of this system are the divers
0:43:00 > 0:43:02who work at these colossal depths.
0:43:05 > 0:43:11This form of extreme diving is called saturation diving.
0:43:11 > 0:43:14When divers descend to these great depths and pressures
0:43:14 > 0:43:16their tissues are filled to capacity
0:43:16 > 0:43:18with dissolved gas in a matter of hours.
0:43:20 > 0:43:21Returning to the surface,
0:43:21 > 0:43:24leaving enough time for all the gas to be released
0:43:24 > 0:43:27and safely avoiding the risk of the bends,
0:43:27 > 0:43:28can take as long as a week.
0:43:31 > 0:43:34That makes short dives completely impractical
0:43:34 > 0:43:37so the only way to operate is for the dives
0:43:37 > 0:43:40to last for a month at a time.
0:43:40 > 0:43:43And they do that by ensuring the divers live at extreme pressure
0:43:43 > 0:43:47even when they're on board the ship.
0:43:47 > 0:43:52Each of these chambers holds two teams of three divers,
0:43:52 > 0:43:54living at 15 times atmospheric pressure.
0:43:54 > 0:43:57The same as being 140 metres underwater.
0:43:59 > 0:44:05This tiny, pressurised habitat will be their home for the next month.
0:44:05 > 0:44:10They eat, sleep and work as if they are deep below the sea.
0:44:10 > 0:44:14Incredibly our bodies can cope with the pressure.
0:44:14 > 0:44:17We're mostly made of water and water can't be compressed.
0:44:17 > 0:44:21The trick to all of this is in what we breathe.
0:44:21 > 0:44:24Now the compressed air that scuba divers use
0:44:24 > 0:44:27rapidly becomes poisonous as you descend further into the deep.
0:44:27 > 0:44:31The nitrogen becomes narcotic, makes you behave in a drunken way.
0:44:31 > 0:44:34But the oxygen... The oxygen is the real problem.
0:44:34 > 0:44:37It becomes horribly toxic. It can injure your lungs
0:44:37 > 0:44:41and can cause potentially fatal convulsions.
0:44:42 > 0:44:44This 1940s film from the US Navy
0:44:44 > 0:44:47shows the effects of breathing oxygen under pressure.
0:44:47 > 0:44:51As more oxygen's forced into the bloodstream,
0:44:51 > 0:44:52it quickly becomes toxic.
0:44:52 > 0:44:57The effects are debilitating and incredibly painful.
0:44:57 > 0:45:00And eventually, though not in this case, fatal.
0:45:02 > 0:45:06So at depth, where the gas has to be under pressure,
0:45:06 > 0:45:08the only way to stop the oxygen becoming toxic
0:45:08 > 0:45:11is to reduce the amount of it we breathe.
0:45:13 > 0:45:17The divers in those chambers are breathing a mixture of gas
0:45:17 > 0:45:20that contains less than 5% oxygen.
0:45:20 > 0:45:22Now, that wouldn't keep them alive here at the surface,
0:45:22 > 0:45:26but because they're diving at such huge pressures,
0:45:26 > 0:45:29it's more than enough for them to live off.
0:45:29 > 0:45:32And then you need to get past the narcotic effects of nitrogen
0:45:32 > 0:45:36and you do that by replacing it with a gas that's more inert,
0:45:36 > 0:45:39less reactive, less harmful to the human body.
0:45:39 > 0:45:42And that gas is helium and that's what you see here
0:45:42 > 0:45:44stored in these huge cylinders.
0:45:44 > 0:45:48Helium mixed with oxygen that's perfect for those divers,
0:45:48 > 0:45:52diving at those huge pressures, to breathe.
0:45:52 > 0:45:58Helium gave the divers something they could breathe at depth.
0:45:58 > 0:46:00It doesn't do them any long-term harm,
0:46:00 > 0:46:04but it does have some very interesting side effects.
0:46:05 > 0:46:06This is the headset here,
0:46:06 > 0:46:10it's got an incorporated mic so if you just stick that on.
0:46:10 > 0:46:12And you can speak to Theo initially.
0:46:12 > 0:46:14All right. Hi, Theo, this is Kevin.
0:46:14 > 0:46:16How long have you guys been in sat for?
0:46:16 > 0:46:20- HIGH-PITCHED VOICE:- Er, we've been in... What's today's date?
0:46:20 > 0:46:21Yeah...
0:46:21 > 0:46:24- 19th, I think. I don't know. - 11 days, I think...
0:46:24 > 0:46:2512 days.
0:46:25 > 0:46:30'To me, the helium in the air makes the divers' voices unintelligible.
0:46:30 > 0:46:35'Amazingly, over time, they've learned to understand one another.'
0:46:36 > 0:46:39Absolutely can't understand a single word of that.
0:46:39 > 0:46:42Maybe I'll try with the help of the electronic unscrambler.
0:46:42 > 0:46:44Right, if we put on the unscrambler now
0:46:44 > 0:46:49- you should be able to understand it so here we go.- So apparently this is going to help you.
0:46:49 > 0:46:51All right. Let's try that again. Hi, hi, hi, hi.
0:46:51 > 0:46:53Theo, how are you?
0:46:53 > 0:46:56- NORMAL VOICE:- Good, and yourself? - Good, good, that's much better.
0:46:56 > 0:46:59So how long have you been down there in sat?
0:46:59 > 0:47:01I think about 10-12 days.
0:47:01 > 0:47:03- 12 days?- Yeah, approximately.
0:47:03 > 0:47:06How long to go now?
0:47:06 > 0:47:10Probably about another... another 10 days. 10, 12 days.
0:47:10 > 0:47:13And you're not fed up of it yet?
0:47:13 > 0:47:15Ah, well... Next question!
0:47:17 > 0:47:21Living at these vast pressures puts a huge physiological strain
0:47:21 > 0:47:23on the divers' bodies.
0:47:23 > 0:47:27In the short-term it can affect the nervous system causing tremors
0:47:27 > 0:47:29and lead to bone disease.
0:47:31 > 0:47:34But the biggest challenges are psychological
0:47:34 > 0:47:37because for all the time they're in the chamber
0:47:37 > 0:47:40the divers are absolutely on their own.
0:47:43 > 0:47:46Even though the divers in these chambers
0:47:46 > 0:47:50are less than an inch away physically,
0:47:50 > 0:47:53they are very, very, very far from help.
0:47:53 > 0:47:57Diving at the depths that they are, it would take more than five days
0:47:57 > 0:48:00to decompress them safely to the surface pressure.
0:48:00 > 0:48:03There's no way of getting them out faster than that.
0:48:03 > 0:48:06You can get people back from the moon more quickly than you can get
0:48:06 > 0:48:09these guys out of these chambers.
0:48:09 > 0:48:11And that just gives you an impression
0:48:11 > 0:48:13of how remote they really are,
0:48:13 > 0:48:16how dangerous this whole endeavour really is.
0:48:18 > 0:48:21Saturation diving technology allows divers to work
0:48:21 > 0:48:25routinely at depths of up to 350 metres.
0:48:26 > 0:48:31But the lessons we've learned from these underwater diving systems
0:48:31 > 0:48:36has helped develop technology for much more extreme environments.
0:48:45 > 0:48:49These astronauts are diving in the vacuum of space.
0:48:51 > 0:48:56Spacewalks are essentially the world's most advanced and demanding dive operations,
0:48:56 > 0:48:59and many of the challenges that the astronauts face
0:48:59 > 0:49:03are the same as those that we encounter under the ocean.
0:49:04 > 0:49:07And there is only one environment on Earth
0:49:07 > 0:49:09where they can prepare for these missions.
0:49:11 > 0:49:15This is NASA's neutral buoyancy laboratory and I've been
0:49:15 > 0:49:18coming here for 14 years, ever since it first opened.
0:49:18 > 0:49:21And I've wanted to get into the water all that time,
0:49:21 > 0:49:24and today I get to dive with astronauts.
0:49:28 > 0:49:32And there it is, it's pretty magnificent.
0:49:32 > 0:49:35Huge volume of water here, inside.
0:49:37 > 0:49:42'At 202 feet long and 40 feet deep, the neutral buoyancy laboratory
0:49:42 > 0:49:46'is one of the largest swimming pools in the world.'
0:49:47 > 0:49:52'And it was built with one purpose in mind,
0:49:52 > 0:49:55'to train astronauts to walk in space.'
0:49:57 > 0:50:00Right now just behind me, the astronauts are getting suited up
0:50:00 > 0:50:04for a six-hour dive in that pool and the reason they're doing that
0:50:04 > 0:50:07is because on Earth, if you want to understand
0:50:07 > 0:50:09what it's like to float in space,
0:50:09 > 0:50:12the best thing you can do is get in there and float in the water.
0:50:12 > 0:50:16That is the closest environment to the environment of outer space that we have here.
0:50:16 > 0:50:18And that's what this entire infrastructure,
0:50:18 > 0:50:21this entire building is set up for.
0:50:24 > 0:50:28Once sealed in their suits and submerged in the pool,
0:50:28 > 0:50:31the astronauts will be neutrally buoyant.
0:50:31 > 0:50:33They will neither sink nor float.
0:50:33 > 0:50:36- DBC, you're on 1EVA? - It's pretty good.
0:50:36 > 0:50:42It's the closest we get on Earth to replicating the weightless conditions of a space walk.
0:50:42 > 0:50:46The bottom of the pool is covered in full scale replicas
0:50:46 > 0:50:49of international space station modules
0:50:49 > 0:50:52where astronauts can rehearse the complicated missions
0:50:52 > 0:50:55they will have to perform in orbit.
0:50:55 > 0:50:58And today NASA are allowing me
0:50:58 > 0:51:02into the pool to observe operations up close.
0:51:02 > 0:51:07It's an incredibly rare opportunity and it's the closest I'll ever get
0:51:07 > 0:51:10to being in orbit around the space station.
0:51:29 > 0:51:31Just incredible to be down here.
0:51:34 > 0:51:35Such a...
0:51:37 > 0:51:39..bizarre place.
0:51:43 > 0:51:47Well, I'm kneeling on top of
0:51:47 > 0:51:50the United States Destiny module,
0:51:50 > 0:51:53the American laboratory.
0:51:53 > 0:51:55In the background there,
0:51:55 > 0:52:00a couple of astronauts going through their paces.
0:52:01 > 0:52:05Practicing procedures that they're going to need
0:52:05 > 0:52:10when they're up there for real on mission.
0:52:13 > 0:52:17Whether in the extreme environments of space or deep underwater,
0:52:17 > 0:52:22we are far from the comfort and safety of the surface of our planet.
0:52:24 > 0:52:28And when these astronauts perform the same operations in space
0:52:28 > 0:52:32they will be living like saturation divers.
0:52:33 > 0:52:37When they emerge from the relative safety of the space station,
0:52:37 > 0:52:40they will be reliant on their suits to protect their bodies
0:52:40 > 0:52:44and provide the oxygen they need to survive.
0:52:48 > 0:52:51It's fantastic to get up close to this stuff,
0:52:51 > 0:52:56to get a sense of what it would feel like to be
0:52:56 > 0:53:01on your own out there, against the void.
0:53:01 > 0:53:04And all of this that you see,
0:53:04 > 0:53:09a stark reminder that this underwater environment
0:53:09 > 0:53:13is such an alien environment,
0:53:13 > 0:53:17that it's as close as we can get here on Earth
0:53:17 > 0:53:19to being in space.
0:53:20 > 0:53:23'The invention of diving systems like these
0:53:23 > 0:53:28'has helped us into the depths of space and the heart of the oceans.
0:53:28 > 0:53:32'But they still can't take us more than a few hundred metres down.
0:53:32 > 0:53:38'To reach the very deepest depths, we've had to invent a different type of technology.
0:53:38 > 0:53:43'This is the HMS Torbay, one of the Royal Navy's fleet
0:53:43 > 0:53:45'of hunter-killer submarines.'
0:53:46 > 0:53:49So welcome to HMS Torbay. This is the control room.
0:53:49 > 0:53:53This is where we operate and run the submarine from.
0:53:53 > 0:53:55We fire all our weapons and we control the depth.
0:53:55 > 0:53:58These are our periscopes, this is what we use to look out
0:53:58 > 0:54:01when we're up at periscope depth or on the surface.
0:54:01 > 0:54:03This is where our planesman sits.
0:54:03 > 0:54:06Much like an aircraft joystick, up and down, left to right.
0:54:06 > 0:54:09He controls our depth and steering of the submarine.
0:54:09 > 0:54:12You imagine that these should be right at the front of the boat.
0:54:12 > 0:54:15- But of course they're not, they're in the middle here.- Yes.
0:54:15 > 0:54:18We don't have any windows on submarines so it doesn't matter where you put it.
0:54:18 > 0:54:21I'll take you down now to the air purification space
0:54:21 > 0:54:24- where we monitor the atmosphere for the submarine.- Perfect, thank you.
0:54:26 > 0:54:31Submarines work by taking a piece of the atmosphere down with them to depth
0:54:31 > 0:54:32and protecting it in a hull
0:54:32 > 0:54:35that can withstand the extreme pressures outside.
0:54:37 > 0:54:41Early submarines had to return to the surface frequently
0:54:41 > 0:54:43to replenish their air supplies.
0:54:43 > 0:54:46But modern subs can stay underwater much longer
0:54:46 > 0:54:50because they can make their own oxygen.
0:54:50 > 0:54:53On the Torbay, it's Lieutenant Commander Simon Murray
0:54:53 > 0:54:57who looks after the air purification system.
0:54:57 > 0:55:02So just tell me, in basic terms
0:55:02 > 0:55:07what all this kit has to do to create an atmosphere that's breathable for your crew.
0:55:07 > 0:55:13It has to replicate the natural balance of elements
0:55:13 > 0:55:16that we have in the air if we were on the surface.
0:55:16 > 0:55:21So to create oxygen for, you know, a crew of dozens and dozens of men,
0:55:21 > 0:55:24what actually do you have to do?
0:55:24 > 0:55:27It's actually very simple. We take demineralised water
0:55:27 > 0:55:30and we pass a DC electrical current through the water
0:55:30 > 0:55:32to split it into hydrogen and oxygen.
0:55:32 > 0:55:36Oxygen we then keep, put back into the atmosphere which we then breathe
0:55:36 > 0:55:39and the hydrogen is compressed and discharged overboard
0:55:39 > 0:55:41cos it's not required.
0:55:41 > 0:55:45You've got your plentiful supply of water and energy.
0:55:45 > 0:55:48How long can that produce an environment
0:55:48 > 0:55:49that your sailors can breathe?
0:55:49 > 0:55:53Actually, indefinitely, we can produce the atmosphere.
0:55:53 > 0:55:56Our only limiting factor on the submarine is the food.
0:55:56 > 0:55:59We carry enough food for a little over three months.
0:55:59 > 0:56:02But everything else... We produce all the oxygen,
0:56:02 > 0:56:06the removal of carbon dioxide and because we have a nuclear reactor
0:56:06 > 0:56:09our fuel for propulsion is indefinite.
0:56:09 > 0:56:13The Torbay can potentially stay underwater forever
0:56:13 > 0:56:15and can dive to immense depths.
0:56:15 > 0:56:18It's exact operational capability is classified
0:56:18 > 0:56:23but it can almost certainly go below 500 metres.
0:56:23 > 0:56:26But that's by no means as deep as some subs can dive.
0:56:26 > 0:56:30And just as Everest was climbed because it was there,
0:56:30 > 0:56:33we have had to journey to the very bottom of the sea.
0:56:35 > 0:56:40On 23rd January 1960, a submarine designed by the Swiss,
0:56:40 > 0:56:43built in Italy and acquired by the US Navy,
0:56:43 > 0:56:45dived to the Mariana Trench.
0:56:45 > 0:56:49At nearly 11,000 metres below sea level
0:56:49 > 0:56:52it's by far and away the most remote and hostile environment
0:56:52 > 0:56:56anywhere on Earth ever visited by a human being.
0:56:58 > 0:57:01In one of the greatest single feats of exploration
0:57:01 > 0:57:05Jacques Piccard and Don Walsh descended into the Mariana Trench,
0:57:05 > 0:57:07in the bathyscaphe Trieste.
0:57:10 > 0:57:13For over five hours they went straight down,
0:57:13 > 0:57:17until, when they'd reached a depth of over six miles,
0:57:17 > 0:57:21with pressure outside 1,000 times what it was at the surface,
0:57:21 > 0:57:23they touched the bottom.
0:57:25 > 0:57:28But no sooner had they arrived than the vast pressure
0:57:28 > 0:57:30caused a window pane to give way,
0:57:30 > 0:57:34shaking the entire vessel as it cracked.
0:57:35 > 0:57:38Fearing the worst, they came straight back up.
0:57:39 > 0:57:40More than 50 years later
0:57:40 > 0:57:44Piccard and Walsh are still the only people to have been
0:57:44 > 0:57:45to the bottom of the Mariana Trench.
0:57:47 > 0:57:49For all the centuries of exploration,
0:57:49 > 0:57:53and the technology that has been developed to explore the seas,
0:57:53 > 0:57:55we have still spent less than 20 minutes
0:57:55 > 0:57:58on the very bottom of the ocean.
0:58:01 > 0:58:05Walsh and Piccard's journey remains one of the most audacious
0:58:05 > 0:58:08extreme expeditions ever undertaken.
0:58:08 > 0:58:12They are the only two people who've ever reached the deepest part of our oceans.
0:58:12 > 0:58:15More people have walked on the surface of the moon
0:58:15 > 0:58:18than have ever got to the bottom of the Mariana Trench.
0:58:18 > 0:58:21And that's the thing about our deep oceans. They are so hostile,
0:58:21 > 0:58:26so alien, so difficult to get to, that they're more similar
0:58:26 > 0:58:31to many parts of outer space than they are our own planet.
0:58:32 > 0:58:36And space is where we're going in the next programme.
0:58:36 > 0:58:39I'm going to find out how we made the journey up
0:58:39 > 0:58:43from the Earth's surface, discover the dangers of altitude,
0:58:43 > 0:58:48and experience how we've been able to scale the highest mountains
0:58:48 > 0:58:50and have mastered the technology to take us
0:58:50 > 0:58:54to the highest reaches of the atmosphere and beyond.
0:59:04 > 0:59:05Subtitles by Red Bee Media Ltd
0:59:05 > 0:59:07E-mail subtitling@bbc.co.uk