Down To Boldly Go

We think of ourselves as a global species.

But our bodies can only survive unsupported on a fraction of the Earth's surface.

We've evolved to live in a temperate climate at sea level,

yet our relentless desire to explore

has pushed us higher and deeper, to every corner of the planet.

I'm Dr Kevin Fong.

I study the limits the human body can endure.

I've worked with NASA for 15 years,

finding ways to keep people alive in orbit.

And now, I'm going to use my own body

to demonstrate what happens...

Just stick with it.

..when we go to the most extreme environments on our planet.

I feel pretty heavy.

Whoa!

And that's gone.

In this programme, I'm going down, beneath the water

to explore our biological limits

and to discover the technology we've had to invent

to take us deeper and deeper.

This is sea level, where we breathe without effort,

without giving it a second thought.

But three quarters of this planet are covered by a body of water

which, for human physiology, represents only threat.

This is the story of how our bodies respond when they are submerged into the water,

and how technology has allowed us to explore the ocean depths.

Water is such a hostile environment

that we can only survive in it for as long as we can make a single breath last.

And exactly how long that is depends on how our bodies react,

because our physiological responses can either extend the time we can stay submerged

or they can limit our survival time to a matter of seconds.

This is the Royal Navy's Helicopter Underwater Escape Training facility.

And it exists because if you're flying in helicopters over open water,

you've got to be prepared for them to get into some difficulty.

And so that is a simulation, a model of a Sea King helicopter.

And today, I'm going to be inside that, upside down in the dark, trying to escape.

All Royal Navy flight crew have to go through this training.

It's designed to teach them the skills they need to escape from a helicopter

after it's crashed into the open water.

And I'm going to take part to demonstrate how one of the body's responses to sudden immersion

can dramatically reduce our chances of survival.

-OK, happy?

-Feels good, yeah.

Feet first, straight into the back of the module.

-Just straight in?

-Straight in.

Head back up against the seat. That is your crash position.

Our survival underwater

ultimately depends on how long we can hold our breath.

On dry land, most of us can manage about a minute.

Break, break, break!

But in a stressful scenario like this,

panicking and disorientated as the helicopter hits the water,

the situation is very different.

My body's first response to the crash is to activate its fight or flight response.

As the fear kicks in, the adrenal glands on top of my kidneys

release a shot of adrenaline into my bloodstream.

This makes my heart beat harder and faster,

sending more blood pumping round my body,

preparing it for a brief period of intense physical activity.

And crucially, this exertion,

coupled with the anxiety,

can severely limit how long we can hold our breath.

On dry land, the fight or flight reflex

improves our chances of survival.

Underwater, it does the opposite.

In these tests, the average time most people can last is less than 20 seconds.

So, it should be pretty straight forward.

Open a window and...

pull yourself out.

In the dark, going under the water...

slightly different prospect!

Not sure I'd want to do that in real life!

Escaping the helicopter is difficult enough,

but there's a much greater threat to our survival.

If you survive a helicopter crash at sea,

then the real killer

is something that the swimming pool just doesn't prepare you for.

And that's the temperature of the water.

The body's response to the cold is so profound

that kills and disables even the strongest swimmers

in a matter of seconds.

To demonstrate this lethal reflex,

I've come to see thermal physiologist, Mike Tipton.

-Come and have a seat.

-Excellent.

We'll get you ready to go.

'Mike's going to plunge me into cold water

'to see what effect it has on my physiology,

'and particularly how it affects my ability to hold my breath.'

What temperature's the water at today, Mike?

It's 12 degrees Celsius,

which is a little bit above the average water temperature around the British Isles.

Wired up to a heart rate monitor,

and with a tube to measure how much air I'm breathing in and out,

I'm ready to be dunked in the freezing tank.

OK, Kevin, are you happy to go?

Well, happy's a strong word, but we'll see how we go.

OK, let's go for it.

Before I go in, my skin temperature is a normal 37 Celsius.

In three, two, one...

go!

Hold the breath, hold your breath.

The cold shock has a massive effect on my body.

As soon as the water touches my skin,

it triggers a dangerous chain of reactions.

My pulse shoots up to over twice its normal rate.

My muscles go into spasm in an attempt to generate more heat,

causing my body to shiver uncontrollably.

And although I'm trying to hold my breath,

I just can't.

Now breathe freely, breathe freely.

Looking good.

Just stick with it.

That's good. Starting to settle down.

That's looking fine.

Take the mouth piece out and have a chat.

You can take your nose clip off as well.

So...

..subjectively, how was that?

Very painful.

Couldn't hold my breath there at all.

Really, really trying,

and I can hold my breath for well over a minute at rest.

Quite shocking, actually. Thought I'd be able to do it.

Really motivated to try and hold my breath there.

After just a couple of minutes in the water,

my skin temperature has plummeted.

But it's the data Mike has collected

that reveals just how lethal the cold shock can be.

We know that when you were completely at rest, when we first sat you in the chair,

you had a heart rate of about 50 beats per minute.

And that's gone up to just over 100, just before you went in.

So that's being driven by very understandable anxiety.

During that first 20 seconds of immersion,

-it was up to around about 145 beats per minute.

-Wow!

That's a remarkably high heart rate for somebody who's essentially sitting still.

But, of course, that's all being driven by those cold receptors that are making your heart beat faster.

It's clear that my pulse is affected by the cold water,

but most shocking of all was my respiration rate

as I'm trying to hold my breath.

There's where you start breath-holding, where you went into the water.

Now we've got the data and we've analysed it, we can see your breath hold time is 12 seconds.

That's still about twice what we would expect somebody wearing the amount of your clothing

to achieve on average, so it's a good performance.

Is that true, that the average person,

wearing no real protective clothing,

holds their breath six seconds?

Five or six seconds is average.

To illustrate how serious this is,

you just need to apply that six-second breath-hold

to a situation like the helicopter crash simulator.

The average person would stand no chance of getting out of the helicopter.

I lasted for another six seconds,

but it's still not enough.

I would still have been underwater

when that uncontrollable urge to take a breath came.

And if you inhale underwater,

the consequences are disastrous.

So now you're breathing uncontrollably,

and you're shifting three, four, five litres in and out of the lung every breath.

And just one-and-a-half of those litres has to be water rather than air

for you to have crossed the lethal dose for drowning.

It's pretty terrifying data to look at

cos this is my 12 second measly 12 second - breath-hold,

and then I take a nearly two-litre breath.

So every single one of these breaths

fills my lungs with a lethal volume of water.

Every single one these breaths!

That kills me. That kills me. That kills me.

Every one of these breaths that I can't control

drowns and kills me.

The cold shock response accounts for something like 60% of all drownings in British waters each year.

It kills strong swimmers, even in calm conditions.

And it's a shocking reminder of just how poorly adapted to water we really are.

Given how utterly hostile water is to our bodies,

you'd think that we'd just try and steer clear of it altogether.

But we don't. We're strangely drawn to it

and the ocean depths, and we try to explore.

When we do, we find surprising aspects of our physiology and biology

that are better adapted to the life aquatic than we think.

Sara Campbell is a prime example of just how well our bodies can adapt to the water.

She's able to stay underwater for longer

and dive much deeper than almost any other human.

It feels like a very natural place for me to be.

For as long as I can remember, even before I learnt how to swim,

I loved being underwater.

I've often questioned where this talent has come from, this ability,

and I just have to remember that I'm a mammal

and we all evolved from the seas millions of years ago.

And for me, it feels like home.

Sara has trained her natural ability

to become a world champion free-diver.

She's able to harness a range of physiological responses

that allow her to survive underwater for extended periods.

And she's promised to teach me the first step,

which is how to hold my breath for longer.

So the first thing that we start with is teaching how to breathe correctly.

Most people have a very shallow breath and they're not maximising the capacity of their lungs.

So what we need to think about is a three part breath.

Starting with the belly,

moving up to the chest

and finishing by topping up into the shoulders.

We usually use only 10% of our lung capacity,

but Sara's technique makes use of their full volume,

preparing her body for a breath-hold attempt.

All right?

Do I need the sound effects as well?

-Try doing it as if you're sucking through a straw.

-OK.

-Yeah?

Yeah. OK, so for the next part, we're going to do a breath-hold.

So we're going to lie you down, do some long, deep breathing, do some relaxation.

All right.

So the most important thing now is for you to relax.

And be nice and calm.

So when you feel ready,

hold your breath.

Give me the signal with a finger that you've begun your breath-hold.

With every breath,

we're inhaling oxygen and exhaling carbon dioxide.

But when we hold our breath,

that gas exchange in our lungs grinds to a halt

and changes our physiology.

The oxygen levels in our blood start falling,

but more importantly, the carbon dioxide levels rise.

And it's this rise in CO2 that's detected by receptors in our brain,

which in turn trigger our urge to breathe.

And one of the first places to feel those signals is the diaphragm,

which starts to contract involuntarily in an attempt to inflate the lungs.

What I have to do is try and ignore those spasms.

Though, of course, you should never try to do this at home.

Here we can see Kevin's diaphragm pulling down...

..wanting to breathe.

That's good. Keep calm, Kevin.

Keep relaxed.

When you feel that urge to breathe, it's not coming from a lack of oxygen,

it's coming from the fact that the carbon dioxide levels in your blood are increasing.

But actually, you have more than enough oxygen in your body

to continue for 30 seconds, one minute, two minutes more.

Three, two, one...

and breathe. Well done.

Good. How do you feel?

Out of breath!

So you've got your contractions at one minute, 30.

And you held to a one minute, 50. So it's a very good start.

Not quite enough for championship free-diving, I don't think.

Not yet! Well done.

What can you manage?

-Oh, I've done 5.12.

-5.12!

But if you compare that to the world records,

the women are at 8 minutes, 20

and the men at 11.35.

-11.35?!

-Mmm.

-Little bit of practice.

-I'm sure that can't be good for you!

But you see how the body is able to adapt and deal with extreme lacks of oxygen

and still survive. It's really incredible.

From her base here in Egypt, Sara has trained her remarkable ability

to become one of the world's best free-divers.

A sport in which the aim is to dive as deep as possible

on a single breath.

It marks the limit of how far we can explore underwater without the aid of technology,

and it pushes human physiology to the very limit.

So Sara's just here,

looking very comfortable at 10 metres,

and this, for her, is just the beginning.

She can manage to get down another 90 metres to almost 100 metres.

And to do that, her body has to undergo

some quite impressive physiological changes.

The first change happens

as soon as the water touches the nerve endings in her face.

In response to the water, signals are sent to the heart,

telling it to slow down,

reducing its work rate and conserving oxygen to be used by the brain.

Let's have a look.

Now that's nice and slow.

It's probably getting on for...

less than 50 beats per minute there.

And that's pretty impressive.

When Sara gets down, closer to 100 metres...

..her heart rate can get down as low as 36 beats per minute.

As she dives deeper, her physiology changes even more.

Her circulation changes.

The blood vessels in her limbs constrict,

reducing blood flow to the body's periphery,

further conserving her oxygen supplies.

At depths below 50 metres,

the pressure can cause fluid to leak into her lungs.

It's a potentially dangerous response,

but it may protect the lungs from further damage on very deep dives.

It's these changes that have allowed Sara to dive straight down for over a minute-and-a-half

to the incredible depth of 96 metres.

This ability to change our physiology underwater

is something we share with all mammals.

It's called the dive reflex,

and it's a throwback to our distant evolutionary past

when we were all aquatic creatures.

Today, the dive reflex is strongest in marine mammals

which are able to survive underwater

because they can reduce their demand for oxygen to almost nothing.

While Sara can dive to nearly 100 metres,

a sperm whale can reach 3,000 metres,

and stay submerged for 90 minutes.

What you have to realise when you're watching Sara do this incredible thing,

diving free on a single breath,

is you forget that this is a limited lease for her.

She's fighting the urge to breathe as the carbon dioxide builds up in her body, in her bloodstream,

and at the same time, the oxygen levels are falling.

So sooner or later, something has to happen.

Either she has to return to the surface to take a breath,

or she's going to pass out.

And the danger of passing out gets greater the closer she gets to the surface.

Already oxygen-starved,

the sudden drop in pressure further reduces the level of oxygen in her bloodstream.

Which can have devastating consequences.

This is Sara diving in the Bahamas,

attempting to become the first woman to dive to 100 metres

on a single breath.

Having reached her target,

all she had to do to claim the record

was return safely to the surface.

Hold yourself up. Breathe, breathe. Nose clip. Nose clip...

-Grab her!

-Sara, Sara, Sara?

Get the weight off.

Blow in her face.

Come on, Sara. Come on, Sara.

Breathe.

Breathe.

Sara, all right.

Without the safety divers on hand,

-Sara could have drowned.

-I'm OK.

As it was, because she passed out, her record didn't stand.

Blackouts like these are common amongst free-divers,

and they reveal our underlying frailty in the water.

Sara is a world class free-diver,

and despite all of her training,

and all the precautions that she takes,

she cannot make this activity completely safe.

She's at the very, very edge of her physiology each time she descends into the deep.

And it's a sobering reminder, reading the text books and the case histories

of people who've had horrific accidents, sometimes fatal accidents,

undertaking exactly this type of feat.

And so our stays underneath the surface of the ocean, unsupported,

are limited to a few hundred seconds, a few tens of metres.

And if we want to stay longer and go deeper,

we're going to need something in the way of technology that can protect us.

We've been inventing technology to take us deeper,

and stay longer underwater, for hundreds of years.

The first devices were diving bells

in which people could descend beneath the waves

in a trapped pocket of air.

Legend has it that as long ago as 300BC,

Alexander the Great was using a giant glass diving bell.

But it wasn't until the 18th century that our ability to explore the depths

was revolutionized by the invention of a diving suit

that gave us the freedom to move around underwater.

'To experience what early diving was like,

'I've come to this ultra-modern aquarium in Cheshire

'to meet Howard Dykes from the Historical Diving Society.'

This is your kit?

'He's going to let me try out his antique diving suit.'

That's an Admiralty pattern 6-bolt standard dress diving helmet.

Until what time were they using all of this?

-Late '60s, early '70s.

-As late as that?

-Yeah.

-They still dive on kit like this in other countries.

-Gosh!

All this is fantastic.

It's one of these things that you have memories of it from childhood,

reading comic books or watching old black and white films.

It's quite incredible to see one up close.

That's why I've started diving.

-Really?

-Yeah, with my Action Man.

-I had my Action Man...

-I remember, yeah!

-..the standard dress diver.

-And this is why I got into diving.

-Yeah, I think it was Tintin for me.

That's it, Red Rackham's Treasure.

Red Rackham's Treasure, that's the one!

Brilliant. I can't believe I'm going to get the chance to put one on!

There's more to the suit than just the helmet.

It comes with a waterproof canvas suit,

a heavy-duty brass collar,

and, of course, matching shoes.

They come in a right and a left, these?

-Yeah, with the buckles on the outside.

-Ah, OK.

And we're all happy that this is all completely safe?

-No.

-No?

OK, great! Brilliant(!)

Well, let's get on then!

Right now I am...

..wearing a...

..very heavy metal collar,

some lead-weighted shoes,

next to a body of water full of sharks.

What could go wrong(?)

-'Hello, diver?'

-Hello.

'Can you hear me OK?'

I can hear you fine, thank you.

And here I go.

First step.

That's really quite heavy.

I am underwater!

See if I can make a step forwards.

That all feels pretty good.

That's great!

And I am diving.

That's really fantastic, actually.

Some of those lovely sharks over there

wondering what the hell it is that this crazy guy is doing.

'In these suits, divers could stay underwater for many hours,

'reaching depths of well over 100m,

'but they were totally dependent on hand-operated pumps on the surface.'

Despite the fact that I'm in a few metres of water,

surrounded by sharks, the thing that most worries me is not them,

it's that that pump being operated by a couple of guys on the surface,

running air down the hose, that's entering my helmet,

that all of that simple but all-important life support

continues to work.

'The deeper I dive, the harder they have to pump

'to maintain the air pressure inside the suit.

'If they stop, I'd soon feel the pressure of the water outside.'

There's an eel. I'll try not to tread on him.

'At greater depths, that crush can be fatal. In the worst cases,

'where the valves in the helmet failed catastrophically,

'the divers' whole bodies were squashed into their helmets.

'A fate delicately known as "the squeeze".'

You come to realise that the whole endeavour of diving

wasn't invented so that someone could have a bit of fun underwater,

it was invented so that we could take ourselves

to this extreme environment...

..and not just visit, but work here.

'It was divers in suits just like this

'who built many of the great Victorian engineering projects -

'the bridges and tunnels that are still used today.'

-RADIO:

-'It's time to come home now.'

Apparently, it's time to go home.

-RADIO:

-'Can the guys stop pumping now?'

Yes, porthole's open.

Thank you, guys. Thanks so much for keeping me alive.

It was brilliant. Really, really fantastic experience.

I was a bit disappointed I didn't get to wander around a bit more.

Very disappointed to come up so soon.

I'll have to come back and do it again another day.

This very simple but very robust piece of equipment

opened up a whole new world for us -

allowed us to do things underwater we'd never been able to do before.

But with all of that advantage came a drawback

and that was an industrial disease,

a disease that would injure people, paralyse them, kill some of them,

and that was the bends.

'The bends is still every diver's worst nightmare.

'To demonstrate what causes it,

'I've come to the Diving Diseases Research Centre in Plymouth.'

'I'm about to go on one of the deepest dives of my life

'but I'm not even going to get wet.

'I'm going diving in a pressure chamber

'which can replicate the conditions of being deep underwater.'

So this is the chamber we're going to be diving in today.

They're going to fill it with compressed air

and get it down to 40m. By the time we're down there,

the pressure in here is going to be five times what it would be at the surface

and, although it's going to look the same, to my body,

my biology, my physiology, it's going to be a whole new world.

-Good to go.

-Understood. That'll be a minute 25, and we're leaving.

'To simulate depth, air is pumped into the chamber to increase the pressure.

'When there's twice as much air inside,

'the pressure has doubled and it's equivalent to being 10m underwater.

'As more air comes in, you can even see the effects.

'Anything with an air pocket in it is squashed.

'If I didn't clear my ears constantly,

'my ear drums would burst.

'It's even too much for some of our cameras.

'By the time five times more air has been pumped into the chamber,

'we're at the same pressure as we would be 40m underwater.

'Every bit of my body is now being squeezed five times harder than it would be at the surface

'and that squeeze is affecting me in some unexpected ways.'

I think I've developed a lisp.

What's that about? I don't like that.

Seriously, why have I got a lisp?

The reason my voice sounds quite so strange, erm...

is because the air is sufficiently dense at this depth

that the speed of sound is faster, the pitch is higher.

And not just that, but two out of three of our cameras have given up,

stopped working, so this is going to be a challenging bit of television.

We're now going to sit here for the next half an hour or so

and hope that our bodies respend...

our bodies, we are going to hope that our bodies respond better

than the cameras do.

'In addition to my squeaky voice, you may have noticed

'that I'm having trouble speaking or even thinking clearly.

'It's because just breathing air at this pressure makes us drunk.

'It's an effect best demonstrated with a bottle of champagne.'

So this is a bottle of champagne, normally very fizzy

and the sort of thing you could have a good celebration with.

Let's just take the cork off this.

And there you go. Very impressively unbubbly bottle of bubbly.

The reason that's happening

and the reason this looks like a very unappetising and flat...

Look at that - a flat glass of champagne.

..is because the pressure is keeping the carbon dioxide

firmly in solution, so...

..this doesn't want to fizz.

'The reason the champagne is flat is that the higher pressure is stopping the bubbles forming

'by forcing the gas to stay dissolved in the liquid.'

Exactly the same thing is happening in my body.

In my lungs, the pressure is forcing the gases in the air to dissolve in my blood.

There's now many times more gas dissolved in my body

than there was at the surface and most of it is nitrogen.

That dissolved nitrogen affects my brain like a powerful narcotic.

It's what divers call "the narcs".

You don't need champagne to get drunk when you're at this depth.

The nitrogen in your body is enough to start to get you drunk.

Nitrogen, which is normally inert at sea level,

begins to behave like an anaesthetic agent at this depth -

starts to make you feel giddy, light-headed,

starts to make you behave slightly abnormally.

And, erm...so, it's just as well, because the alcohol tastes rubbish.

'While the narcs are a lot of fun in the controlled environment of the chamber,

'they can be very dangerous underwater.

'The only way to relieve the symptoms is to come to a shallower depth.

'But it's on the return to the surface that the danger of the bends begins.

'As the pressure is reduced,

'the dissolved gas begins to come out of solution again.

'It's vitally important that we come up slowly

'to allow the extra nitrogen in my body to be released gradually through the lungs.

'If we were to come up too quickly

'the gas would suddenly come out of solution, forming bubbles

'in my tissues and blood vessels like bubbles in champagne.

'It's those bubbles that cause the bends

'and they can have devastating consequences.'

These are the worst consequences of an attack of the bends.

This is what happens when the bubbles that evolve in your tissues and blood stream get in

and block vessels either by passing through and causing damage

or lodging in those vessels.

These two pictures are people with brain damage.

The white spots you can see here are bits of dead brain -

brain that's died because it has lost its blood supply.

Down here you're seeing someone who's had bubbles evolve in their bone marrow

and all those black spots you can see there are areas of expanded gas

that are going to cause a lot of pain and problem later on.

Most alarmingly, this up here,

a shot of brain in a post mortem,

and you can actually see bubbles here

expanded in the veins around the brain

that have caused a blockage and contributed to this patient's death.

These are lethal or potentially lethal injuries.

'With the risks of drowning and the bends,

'it makes you wonder why anyone would want go diving.

'But our inherent desire to keep pushing the boundaries

'means millions and millions of people around the world

'now count diving as a hobby.'

'The reason we can do that is all down to one remarkable invention.'

It was this type of equipment, just over half a century ago,

that revolutionised the underwater experience.

We take them for granted now because we see them everywhere

but this bit of kit has two very important components.

A tank of compressed air so you could take your air supply with you and not depend on the surface,

and then, secondly, this pressure-reducing demand valve.

That brought this very high pressure down to something you could breathe

but, also, it allowed you to draw air off on demand

and allowed this finite supply of air to last much longer.

'It was pioneered by French marine biologist, Jacques-Yves Cousteau.

'Carrying our own air supply gave everyone the freedom of the ocean.

'It allowed us to dive independently for more than an hour at a time

'and reach depths of 30m and beyond in relative safety.

'Since Cousteau's time, scuba technology has come a long way.

'Modern divers carry much more equipment and gadgetry

'but the basic principle has remained unchanged.'

30 metres. Very, very, very beautiful.

All of this remarkable engineering,

this self-contained underwater breathing apparatus,

allows us to explore underwater freely.

But it only just gives us access to the first few tens of metres.

Out there is an entire ocean that I can't get near dressed like this.

For that, you need much more substantial equipment.

With basic scuba gear,

we're limited to diving a few tens of metres below the surface.

But we want and need to go much deeper than that.

This is the North Sea.

Its floor is home to some of the world's richest deposits

of oil and minerals,

but to get to those deposits we have to put men on the sea floor,

often over 100 metres below the surface.

And doing that is a massive technical and physiological challenge.

This is the Toisa Polaris, a very impressive-looking dive support vessel.

This vast ship complete with helicopter pad, cranes,

100 crew, it's all here to support just a handful of men

diving in the North Sea.

To reach the bottom of the sea, the divers take a diving bell.

An air-filled capsule pressurised to up to 35 times atmospheric pressure.

Chad, what is this place?

This here is our dive bell hanger and this here is the dive bell.

So how does that get into the ocean?

We trolley it over, you can see the trolley tracks.

Oh, right, OK.

Then we come over here to the moon pool and then it descends down.

So that's kind of like your taxi to work?

Pretty much, pretty much.

The diving bells provide a vital lifeline to the divers in the water.

An umbilical cord which connects the ship to the bell

and the bell to the divers' suits.

It supplies hot water for warmth, air to breathe, power for light

and communications to the surface.

-Can we have a look inside?

-Definitely.

How long does it take you to reach the bottom

once you come off the ship

and you're going down in one of these?

Well, there's a few variables, depending how deep we're diving

and how bad the weather is

but usually it'll take from 20 to 30 minutes.

Wow!

It's a crazy-looking place. Really cramped in there,

you can't imagine that three people occupy it.

One of those poor blokes has to stay in for the whole six-hour dive.

Once at the bottom, the divers leave the bell wearing a diving helmet

strong enough to withstand the immense pressures.

But impressive as all this hardware is,

the really astonishing aspects of this system are the divers

who work at these colossal depths.

This form of extreme diving is called saturation diving.

When divers descend to these great depths and pressures

their tissues are filled to capacity

with dissolved gas in a matter of hours.

Returning to the surface,

leaving enough time for all the gas to be released

and safely avoiding the risk of the bends,

can take as long as a week.

That makes short dives completely impractical

so the only way to operate is for the dives

to last for a month at a time.

And they do that by ensuring the divers live at extreme pressure

even when they're on board the ship.

Each of these chambers holds two teams of three divers,

living at 15 times atmospheric pressure.

The same as being 140 metres underwater.

This tiny, pressurised habitat will be their home for the next month.

They eat, sleep and work as if they are deep below the sea.

Incredibly our bodies can cope with the pressure.

We're mostly made of water and water can't be compressed.

The trick to all of this is in what we breathe.

Now the compressed air that scuba divers use

rapidly becomes poisonous as you descend further into the deep.

The nitrogen becomes narcotic, makes you behave in a drunken way.

But the oxygen... The oxygen is the real problem.

It becomes horribly toxic. It can injure your lungs

and can cause potentially fatal convulsions.

This 1940s film from the US Navy

shows the effects of breathing oxygen under pressure.

As more oxygen's forced into the bloodstream,

it quickly becomes toxic.

The effects are debilitating and incredibly painful.

And eventually, though not in this case, fatal.

So at depth, where the gas has to be under pressure,

the only way to stop the oxygen becoming toxic

is to reduce the amount of it we breathe.

The divers in those chambers are breathing a mixture of gas

that contains less than 5% oxygen.

Now, that wouldn't keep them alive here at the surface,

but because they're diving at such huge pressures,

it's more than enough for them to live off.

And then you need to get past the narcotic effects of nitrogen

and you do that by replacing it with a gas that's more inert,

less reactive, less harmful to the human body.

And that gas is helium and that's what you see here

stored in these huge cylinders.

Helium mixed with oxygen that's perfect for those divers,

diving at those huge pressures, to breathe.

Helium gave the divers something they could breathe at depth.

It doesn't do them any long-term harm,

but it does have some very interesting side effects.

This is the headset here,

it's got an incorporated mic so if you just stick that on.

And you can speak to Theo initially.

All right. Hi, Theo, this is Kevin.

How long have you guys been in sat for?

-HIGH-PITCHED VOICE:

-Er, we've been in... What's today's date?

Yeah...

-19th, I think. I don't know.

-11 days, I think...

12 days.

'To me, the helium in the air makes the divers' voices unintelligible.

'Amazingly, over time, they've learned to understand one another.'

Absolutely can't understand a single word of that.

Maybe I'll try with the help of the electronic unscrambler.

Right, if we put on the unscrambler now

-you should be able to understand it so here we go.

-So apparently this is going to help you.

All right. Let's try that again. Hi, hi, hi, hi.

Theo, how are you?

-NORMAL VOICE:

-Good, and yourself?

-Good, good, that's much better.

So how long have you been down there in sat?

I think about 10-12 days.

-12 days?

-Yeah, approximately.

How long to go now?

Probably about another... another 10 days. 10, 12 days.

And you're not fed up of it yet?

Ah, well... Next question!

Living at these vast pressures puts a huge physiological strain

on the divers' bodies.

In the short-term it can affect the nervous system causing tremors

and lead to bone disease.

But the biggest challenges are psychological

because for all the time they're in the chamber

the divers are absolutely on their own.

Even though the divers in these chambers

are less than an inch away physically,

they are very, very, very far from help.

Diving at the depths that they are, it would take more than five days

to decompress them safely to the surface pressure.

There's no way of getting them out faster than that.

You can get people back from the moon more quickly than you can get

these guys out of these chambers.

And that just gives you an impression

of how remote they really are,

how dangerous this whole endeavour really is.

Saturation diving technology allows divers to work

routinely at depths of up to 350 metres.

But the lessons we've learned from these underwater diving systems

has helped develop technology for much more extreme environments.

These astronauts are diving in the vacuum of space.

Spacewalks are essentially the world's most advanced and demanding dive operations,

and many of the challenges that the astronauts face

are the same as those that we encounter under the ocean.

And there is only one environment on Earth

where they can prepare for these missions.

This is NASA's neutral buoyancy laboratory and I've been

coming here for 14 years, ever since it first opened.

And I've wanted to get into the water all that time,

and today I get to dive with astronauts.

And there it is, it's pretty magnificent.

Huge volume of water here, inside.

'At 202 feet long and 40 feet deep, the neutral buoyancy laboratory

'is one of the largest swimming pools in the world.'

'And it was built with one purpose in mind,

'to train astronauts to walk in space.'

Right now just behind me, the astronauts are getting suited up

for a six-hour dive in that pool and the reason they're doing that

is because on Earth, if you want to understand

what it's like to float in space,

the best thing you can do is get in there and float in the water.

That is the closest environment to the environment of outer space that we have here.

And that's what this entire infrastructure,

this entire building is set up for.

Once sealed in their suits and submerged in the pool,

the astronauts will be neutrally buoyant.

They will neither sink nor float.

-DBC, you're on 1EVA?

-It's pretty good.

It's the closest we get on Earth to replicating the weightless conditions of a space walk.

The bottom of the pool is covered in full scale replicas

of international space station modules

where astronauts can rehearse the complicated missions

they will have to perform in orbit.

And today NASA are allowing me

into the pool to observe operations up close.

It's an incredibly rare opportunity and it's the closest I'll ever get

to being in orbit around the space station.

Just incredible to be down here.

Such a...

..bizarre place.

Well, I'm kneeling on top of

the United States Destiny module,

the American laboratory.

In the background there,

a couple of astronauts going through their paces.

Practicing procedures that they're going to need

when they're up there for real on mission.

Whether in the extreme environments of space or deep underwater,

we are far from the comfort and safety of the surface of our planet.

And when these astronauts perform the same operations in space

they will be living like saturation divers.

When they emerge from the relative safety of the space station,

they will be reliant on their suits to protect their bodies

and provide the oxygen they need to survive.

It's fantastic to get up close to this stuff,

to get a sense of what it would feel like to be

on your own out there, against the void.

And all of this that you see,

a stark reminder that this underwater environment

is such an alien environment,

that it's as close as we can get here on Earth

to being in space.

'The invention of diving systems like these

'has helped us into the depths of space and the heart of the oceans.

'But they still can't take us more than a few hundred metres down.

'To reach the very deepest depths, we've had to invent a different type of technology.

'This is the HMS Torbay, one of the Royal Navy's fleet

'of hunter-killer submarines.'

So welcome to HMS Torbay. This is the control room.

This is where we operate and run the submarine from.

We fire all our weapons and we control the depth.

These are our periscopes, this is what we use to look out

when we're up at periscope depth or on the surface.

This is where our planesman sits.

Much like an aircraft joystick, up and down, left to right.

He controls our depth and steering of the submarine.

You imagine that these should be right at the front of the boat.

-But of course they're not, they're in the middle here.

-Yes.

We don't have any windows on submarines so it doesn't matter where you put it.

I'll take you down now to the air purification space

-where we monitor the atmosphere for the submarine.

-Perfect, thank you.

Submarines work by taking a piece of the atmosphere down with them to depth

and protecting it in a hull

that can withstand the extreme pressures outside.

Early submarines had to return to the surface frequently

to replenish their air supplies.

But modern subs can stay underwater much longer

because they can make their own oxygen.

On the Torbay, it's Lieutenant Commander Simon Murray

who looks after the air purification system.

So just tell me, in basic terms

what all this kit has to do to create an atmosphere that's breathable for your crew.

It has to replicate the natural balance of elements

that we have in the air if we were on the surface.

So to create oxygen for, you know, a crew of dozens and dozens of men,

what actually do you have to do?

It's actually very simple. We take demineralised water

and we pass a DC electrical current through the water

to split it into hydrogen and oxygen.

Oxygen we then keep, put back into the atmosphere which we then breathe

and the hydrogen is compressed and discharged overboard

cos it's not required.

You've got your plentiful supply of water and energy.

How long can that produce an environment

that your sailors can breathe?

Actually, indefinitely, we can produce the atmosphere.

Our only limiting factor on the submarine is the food.

We carry enough food for a little over three months.

But everything else... We produce all the oxygen,

the removal of carbon dioxide and because we have a nuclear reactor

our fuel for propulsion is indefinite.

The Torbay can potentially stay underwater forever

and can dive to immense depths.

It's exact operational capability is classified

but it can almost certainly go below 500 metres.

But that's by no means as deep as some subs can dive.

And just as Everest was climbed because it was there,

we have had to journey to the very bottom of the sea.

On 23rd January 1960, a submarine designed by the Swiss,

built in Italy and acquired by the US Navy,

dived to the Mariana Trench.

At nearly 11,000 metres below sea level

it's by far and away the most remote and hostile environment

anywhere on Earth ever visited by a human being.

In one of the greatest single feats of exploration

Jacques Piccard and Don Walsh descended into the Mariana Trench,

in the bathyscaphe Trieste.

For over five hours they went straight down,

until, when they'd reached a depth of over six miles,

with pressure outside 1,000 times what it was at the surface,

they touched the bottom.

But no sooner had they arrived than the vast pressure

caused a window pane to give way,

shaking the entire vessel as it cracked.

Fearing the worst, they came straight back up.

More than 50 years later

Piccard and Walsh are still the only people to have been

to the bottom of the Mariana Trench.

For all the centuries of exploration,

and the technology that has been developed to explore the seas,

we have still spent less than 20 minutes

on the very bottom of the ocean.

Walsh and Piccard's journey remains one of the most audacious

extreme expeditions ever undertaken.

They are the only two people who've ever reached the deepest part of our oceans.

More people have walked on the surface of the moon

than have ever got to the bottom of the Mariana Trench.

And that's the thing about our deep oceans. They are so hostile,

so alien, so difficult to get to, that they're more similar

to many parts of outer space than they are our own planet.

And space is where we're going in the next programme.

I'm going to find out how we made the journey up

from the Earth's surface, discover the dangers of altitude,

and experience how we've been able to scale the highest mountains

and have mastered the technology to take us

to the highest reaches of the atmosphere and beyond.

Subtitles by Red Bee Media Ltd

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