0:00:02 > 0:00:07On today's Fierce Earth, it's all about the awesome power of the sea.
0:00:07 > 0:00:10Here, in Santa Cruz, California, home of monster waves.
0:00:12 > 0:00:17And we have monsters here in the UK - monster tides.
0:00:17 > 0:00:20We'll find out how the sea goes from this...
0:00:20 > 0:00:22..To this. It's incredible!
0:00:28 > 0:00:31It's wet suits on for the Fierce Earth team, as we hit the surf
0:00:31 > 0:00:34to unlock the secrets of king-sized waves.
0:00:39 > 0:00:43Leah experiences one of the world's fastest rising tides.
0:00:43 > 0:00:46It's starting to get a little bit scary, actually.
0:00:46 > 0:00:47Remember to stay calm.
0:00:47 > 0:00:51I meet the awesome power of a rip current head on.
0:00:52 > 0:00:55- Good job, Leo, keep it up! - I'm exhausted...
0:00:57 > 0:01:02And Captain Dougal rides the tides to guide a massive ship into port.
0:01:03 > 0:01:06That's one incredible sight to see.
0:01:06 > 0:01:10What happens when the ground shakes,
0:01:10 > 0:01:14the seas rise up and the air tears itself apart?
0:01:14 > 0:01:16The Fierce Earth team move in,
0:01:16 > 0:01:19taking on the most powerful forces on the planet.
0:01:37 > 0:01:38Get ready for Fierce Earth -
0:01:38 > 0:01:40the Earth, and how to survive it.
0:01:43 > 0:01:47The world's shorelines are in a never-ending battle with the sea.
0:01:47 > 0:01:49Battered by the huge force of the waves,
0:01:49 > 0:01:52year in year out, 24 hours a day.
0:01:53 > 0:01:56In the UK, the storms of Christmas 2013
0:01:56 > 0:01:59changed the shape of the coast for ever.
0:01:59 > 0:02:02Around the planet more than 3 billion people live by the sea.
0:02:02 > 0:02:05Waves can be dangerous and even cost lives,
0:02:05 > 0:02:08but when the sea is safe, they can be a lot of fun.
0:02:08 > 0:02:10We're here in Santa Cruz, California,
0:02:10 > 0:02:13home to some of the most famous coastline in the world.
0:02:13 > 0:02:15As you can see, it's pretty popular with surfers -
0:02:15 > 0:02:18that's because the waves here can be absolutely massive.
0:02:18 > 0:02:20We're here to find out why.
0:02:25 > 0:02:28People come from all over the world to ride these waves,
0:02:28 > 0:02:30which can be up to 15 metres tall -
0:02:30 > 0:02:33that's as high as a five-storey building.
0:02:35 > 0:02:38This place is paradise for thrill-seekers -
0:02:38 > 0:02:43making this beach one of the best surfing locations in the world.
0:02:43 > 0:02:45To understand how waves bring the sea to life,
0:02:45 > 0:02:49we first need to know what one is, and how it moves.
0:02:51 > 0:02:54OK, let it go on the ground...
0:02:54 > 0:02:56Whenever we think of waves, we think of the sea,
0:02:56 > 0:02:58but, in fact, waves are all around us.
0:02:58 > 0:03:01I can create my own with this skipping rope. Watch.
0:03:04 > 0:03:06Notice when Leo flicks his wrist,
0:03:06 > 0:03:09the energy transfers along the rope in the form of a wave.
0:03:13 > 0:03:16And that's exactly what happens when waves move through the sea.
0:03:22 > 0:03:25Waves are energy passing through water,
0:03:25 > 0:03:28and sometimes travelling for thousands of miles.
0:03:31 > 0:03:33The next piece of land in that direction is Japan,
0:03:33 > 0:03:35more than 5,000 miles away.
0:03:35 > 0:03:38Some of these waves have travelled almost that far.
0:03:38 > 0:03:40That distance is important in creating the huge waves
0:03:40 > 0:03:42that crash against this coast.
0:03:42 > 0:03:43If you look out there,
0:03:43 > 0:03:47you can just see Clare bobbing up and down on a boat.
0:03:47 > 0:03:49Hi, Clare!
0:03:49 > 0:03:51Hi, Leo! Although it looks like the water itself
0:03:51 > 0:03:53is moving towards Leo on the shore,
0:03:53 > 0:03:58a wave is actually ENERGY passing through the water.
0:03:58 > 0:04:00You can see the boat bobbing up and down
0:04:00 > 0:04:03as the energy of the wave passes underneath it,
0:04:03 > 0:04:05then make its way to shore where it breaks.
0:04:05 > 0:04:08OK, Clare, you can come in!
0:04:10 > 0:04:13So, where does this energy come from?
0:04:13 > 0:04:14Remember the skipping rope?
0:04:14 > 0:04:17Leo started that wave with a flick of his wrist.
0:04:17 > 0:04:19Well, out at sea, waves start with wind.
0:04:23 > 0:04:26Imagine this swimming pool is the Pacific Ocean.
0:04:29 > 0:04:33We're going to use a leaf blower to create a storm with high winds.
0:04:35 > 0:04:38Now, let's send Leo over to the other side of the ocean.
0:04:39 > 0:04:42OK, Clare, I'm in California.
0:04:43 > 0:04:44Now it's the fun part.
0:04:44 > 0:04:49I'm going to start a mini storm and send it in Leo's direction.
0:04:49 > 0:04:51You can see how rough the seas are,
0:04:51 > 0:04:54but as the waves move away from the storm,
0:04:54 > 0:04:57they become organised lines moving across the ocean
0:04:57 > 0:04:59towards the shoreline.
0:04:59 > 0:05:01You can see the energy from the storm
0:05:01 > 0:05:03that Clare has created over there
0:05:03 > 0:05:06has transferred across the pool in the form of these waves.
0:05:07 > 0:05:10So, it's wind that fuels waves.
0:05:13 > 0:05:15When a storm whips up out in the ocean,
0:05:15 > 0:05:17the longer and harder it blows,
0:05:17 > 0:05:20the more energy it propels towards the shore.
0:05:21 > 0:05:25A wave's long journey finally ends near the coast when it breaks,
0:05:25 > 0:05:27but this can only happen
0:05:27 > 0:05:30when the final piece of the jigsaw falls into place.
0:05:33 > 0:05:35As the ocean gets shallower near to the shore,
0:05:35 > 0:05:39the wave slows down due to the friction of the sea bed.
0:05:40 > 0:05:42As the water begins to mound up,
0:05:42 > 0:05:45the wave takes on its distinctive shape.
0:05:45 > 0:05:48Once it gets too tall, it breaks and rolls up the beach.
0:06:00 > 0:06:06At Santa Cruz, California, all these ingredients come together perfectly.
0:06:06 > 0:06:09Thousands of miles of ocean to build up all that energy,
0:06:09 > 0:06:10and then the perfect sea bed
0:06:10 > 0:06:13to create monster breakers like these.
0:06:18 > 0:06:22It's no wonder one sport has made its home here.
0:06:22 > 0:06:24Surfing.
0:06:27 > 0:06:31Leo and I are here to have a go, and we've got two very special teachers.
0:06:37 > 0:06:42Sam, 12, and Ben, 15, have lived next to the ocean all their lives.
0:06:46 > 0:06:51They've been surfing since they were five and are now Junior Champions.
0:06:52 > 0:06:54They're pretty good.
0:06:55 > 0:06:59Their challenge today is to get Clare and I doing this...
0:07:00 > 0:07:01..Well, maybe.
0:07:07 > 0:07:10- I'm Leo.- Ben.- Nice to meet you, Ben. - Sam.- Hi, Sam, how you doing?
0:07:10 > 0:07:14- So, you're going to teach us how to surf?- Yeah, it should be fun.
0:07:14 > 0:07:17- So, do you guys surf every day? - Yeah.
0:07:17 > 0:07:19And what makes it so fun
0:07:19 > 0:07:22that you can surf on the coldest day in winter?
0:07:22 > 0:07:24You just get good waves
0:07:24 > 0:07:27and it gets you happy when you do good turns.
0:07:28 > 0:07:31By following the surf forecasts online,
0:07:31 > 0:07:35the boys know in advance what the sea conditions are going to be like.
0:07:35 > 0:07:38But sometimes this sea gets dangerous,
0:07:38 > 0:07:41and that's where years of experience come in.
0:07:41 > 0:07:44It's important to understand where to be,
0:07:44 > 0:07:47so you don't get caught in a dangerous situation.
0:07:47 > 0:07:49And also you just never want to turn your back to the ocean
0:07:49 > 0:07:53because you'll never know if, like, a big wave is going to take you out.
0:07:53 > 0:07:55- Trying to read the waves. - Your board doesn't want to be
0:07:55 > 0:07:59between you and the wave or, like, it's going to hit you.
0:08:00 > 0:08:03Today, the sea is fairly calm, but the waves off this coast
0:08:03 > 0:08:06are capable of smashing boats and surfboards in two
0:08:06 > 0:08:08with their awesome force.
0:08:08 > 0:08:10There's no way you'd get me out in a sea like that!
0:08:15 > 0:08:18If you want to harness the energy of a wave on a surfboard,
0:08:18 > 0:08:21you have to paddle yourself up to speed
0:08:21 > 0:08:24and leap from a lying position to standing up.
0:08:24 > 0:08:26Sam and Ben make it look very easy.
0:08:26 > 0:08:29Something tells me it isn't...
0:08:31 > 0:08:34So, if you're laying down on the board like this...paddling...
0:08:34 > 0:08:37whatever foot you put forward...
0:08:37 > 0:08:39I don't think I could even do that.
0:08:39 > 0:08:40Let's have a go.
0:08:40 > 0:08:43I don't think we're going to be the next World Champions!
0:08:46 > 0:08:48Too far forward, your nose is going to dig into the water
0:08:48 > 0:08:52- and you'll face-plant. - Woo-hoo! Going to be fun!
0:08:54 > 0:08:58You're getting there, but you're going to need a lot of practice
0:08:58 > 0:09:02- before you get in the ocean. - 'I think he may be right.'
0:09:02 > 0:09:04OK, nice one.
0:09:04 > 0:09:08But, somehow, we persuade them to let us loose in the ocean.
0:09:08 > 0:09:12We head off to a beginners' beach where the boys learned to surf.
0:09:13 > 0:09:16Conditions are perfect, waves of about 1.5 metres.
0:09:17 > 0:09:19Easy for Sam and Ben,
0:09:19 > 0:09:22but they look like skyscrapers to us as we paddle out.
0:09:27 > 0:09:31After a few failed attempts to catch a wave, we're going nowhere.
0:09:35 > 0:09:38But, eventually, with a friendly shove from Ben,
0:09:38 > 0:09:40I get up on the wave.
0:09:40 > 0:09:41Sort of..
0:09:41 > 0:09:45The energy pushes me hundreds of metres into the shore,
0:09:45 > 0:09:47it's an amazing feeling.
0:09:48 > 0:09:52Now, it's Leo's turn, and he has to go one better and stand up.
0:09:55 > 0:09:56Show off!
0:10:03 > 0:10:07- Nice job.- You guys were amazing.
0:10:07 > 0:10:10Without you two I just wouldn't have even got in the water.
0:10:10 > 0:10:13Yeah, you did really good. That one, you got in it.
0:10:13 > 0:10:17Sam, you're amazing! 12-year-old kid teaching me how to surf.
0:10:17 > 0:10:19- You rock!- Got it first time!
0:10:21 > 0:10:25I can't believe the immense power of those waves.
0:10:25 > 0:10:26It was just incredible.
0:10:26 > 0:10:29And to think that thing travelled all the way across the ocean
0:10:29 > 0:10:32and just for that split second, you get to ride that force.
0:10:32 > 0:10:35- It's amazing, isn't it? - It's better than that paddling!
0:10:35 > 0:10:36I want to go and get another one!
0:10:36 > 0:10:39- High-five!- High-fives back!
0:10:39 > 0:10:43Still to come, Leo sees if he has what it takes to beat
0:10:43 > 0:10:46the ocean's hidden menace - the rip current.
0:10:47 > 0:10:49Feels like I'm going nowhere!
0:10:51 > 0:10:54Oceans cover 71% of the Earth's surface
0:10:54 > 0:10:57and contain 97% of the planet's water.
0:10:57 > 0:11:00The energy carried by waves means lots of fun for surfers
0:11:00 > 0:11:04and, as we now know, waves don't actually move the water.
0:11:04 > 0:11:07Think about the last time you went to the seaside.
0:11:07 > 0:11:11Throughout the day, the water level changes
0:11:11 > 0:11:13causing the beach to shrink and grow.
0:11:13 > 0:11:15This is the tide coming in and out.
0:11:17 > 0:11:216,000 miles away from Santa Cruz in California is the UK,
0:11:21 > 0:11:24a place where tides are a very big deal.
0:11:26 > 0:11:28We're in South Wales.
0:11:28 > 0:11:30Behind us is the Severn Estuary,
0:11:30 > 0:11:32the mouth of Britain's longest river.
0:11:32 > 0:11:36The harbour here at the moment is completely full of water.
0:11:36 > 0:11:40But if we were just to wait another six hours,
0:11:40 > 0:11:43the sea level's going to drop an incredible 12 metres.
0:11:46 > 0:11:48To this. It's incredible.
0:11:48 > 0:11:52All of this variation in sea level is due to tides.
0:11:52 > 0:11:54And we're here to find out how they work.
0:11:58 > 0:12:01Tides are one of the great forces on our planet.
0:12:01 > 0:12:04Every day, most coastlines
0:12:04 > 0:12:06experience two high tides and two low tides.
0:12:09 > 0:12:13The driving force behind this constant change is gravity.
0:12:17 > 0:12:21What's going to happen if I throw this ball into the sky?
0:12:22 > 0:12:25Is it going to stay there, is it going to go upwards?
0:12:25 > 0:12:27No, we know what's going to happen.
0:12:27 > 0:12:29It's going to fall to the ground.
0:12:29 > 0:12:33That's because it's being pulled by a force called gravity
0:12:33 > 0:12:35drawing us all into the centre of the Earth.
0:12:35 > 0:12:38It keeps me standing here, it's dropped the ball to the ground
0:12:38 > 0:12:40and it also keeps the oceans in place.
0:12:43 > 0:12:47But we also know the oceans don't stay exactly where they are.
0:12:47 > 0:12:49They are constantly on the move,
0:12:49 > 0:12:52coming in and going out with the motion of the tides.
0:12:52 > 0:12:54Now, why is that?
0:12:54 > 0:12:58If you imagine this is the Earth with its own gravitational force,
0:12:58 > 0:13:01there's something missing here
0:13:01 > 0:13:03and that's up there in the sky.
0:13:03 > 0:13:04The moon.
0:13:05 > 0:13:09The moon travels round the Earth in one month
0:13:09 > 0:13:13and the moon, as a planet, has its own gravitational force
0:13:13 > 0:13:16but it's much weaker than that of the bigger Earth,
0:13:16 > 0:13:19but it's still strong enough to affect the oceans.
0:13:19 > 0:13:24As the moon moves around the Earth throughout each month,
0:13:24 > 0:13:28it pulls the oceans slightly towards it, helping to cause the tides.
0:13:28 > 0:13:32But there's another thing to think about.
0:13:32 > 0:13:34Imagine this is us on the Earth.
0:13:34 > 0:13:36The Earth is constantly spinning,
0:13:36 > 0:13:39giving us one day and one night every 24 hours.
0:13:41 > 0:13:43So, every 24 hours the Earth moves twice
0:13:43 > 0:13:46through the deep water giving us high tides,
0:13:46 > 0:13:49and twice through the shallow water giving us low tides.
0:13:51 > 0:13:54The difference between the height of a high tide
0:13:54 > 0:13:57and a low tide is called a tidal range.
0:13:57 > 0:14:01Almost nowhere in the world sees a bigger tidal range
0:14:01 > 0:14:03than here in the Severn Estuary.
0:14:03 > 0:14:06That's because of its distinctive funnel shape.
0:14:06 > 0:14:09If you've got Wales here and England there,
0:14:09 > 0:14:14the tide is being forced up this funnel-shape river mouth, like so.
0:14:14 > 0:14:16There's only one thing going to happen.
0:14:19 > 0:14:23The water mass is moving in and in as the tide rises,
0:14:23 > 0:14:26but it's getting funnelled in and there's land either side -
0:14:26 > 0:14:28it can't go anywhere,
0:14:28 > 0:14:30so it builds up, just like this sand here,
0:14:30 > 0:14:32and builds up and builds up and builds up.
0:14:32 > 0:14:34So, around about this part of the estuary,
0:14:34 > 0:14:37that's where you get the highest tides,
0:14:37 > 0:14:39because it's being forced into this funnel shape.
0:14:42 > 0:14:46This massive tidal range can be used to great effect by ports,
0:14:46 > 0:14:48because it means, at high tide,
0:14:48 > 0:14:51you can get enormous ships in and out of docks,
0:14:51 > 0:14:53like this one in Bristol.
0:14:53 > 0:14:55But you don't have long to do it.
0:14:59 > 0:15:02Every day, thousands of tonnes of cargo
0:15:02 > 0:15:05come in and out of this massive dock.
0:15:05 > 0:15:07They can bring anything from cars,
0:15:07 > 0:15:09coal, even toys, to Britain.
0:15:10 > 0:15:13But getting the huge ships that carry all of this
0:15:13 > 0:15:18in and out of the docks is a real assault course because of the tides.
0:15:18 > 0:15:21By measuring the distance of the moon from the Earth
0:15:21 > 0:15:23and understanding the shape of the sea bed,
0:15:23 > 0:15:26scientists can predict for many years ahead
0:15:26 > 0:15:28when we will have high and low tides.
0:15:30 > 0:15:34This channel controls the shipping in and out of the dock.
0:15:34 > 0:15:37Now, the level of water in the dock always stays high
0:15:37 > 0:15:41because of these giant gates, they lock the water back.
0:15:41 > 0:15:43But at low tide, we've got a problem.
0:15:43 > 0:15:44Look at the other side here.
0:15:44 > 0:15:48You can see the water level's down to less than a metre,
0:15:48 > 0:15:50you can see mud banks over there.
0:15:50 > 0:15:53There's no way we're going to get a large ship into the docks
0:15:53 > 0:15:55when the tide is low.
0:15:55 > 0:15:57There's only one way to do that -
0:15:57 > 0:15:59wait until the tide has risen high enough
0:15:59 > 0:16:01and very carefully guide the ship in.
0:16:03 > 0:16:05I'm here to meet Jerry.
0:16:05 > 0:16:08- Hello!- Nice to see you. Welcome aboard.- Great!
0:16:08 > 0:16:11He's like an air-traffic controller for ships.
0:16:11 > 0:16:14His job is to get as many ships into port as possible
0:16:14 > 0:16:17during the small window of opportunity.
0:16:17 > 0:16:19He needs to know exactly when the tide will be high
0:16:19 > 0:16:22and when it will be low.
0:16:22 > 0:16:23Harbour control...
0:16:23 > 0:16:28We're heading out into the estuary to meet a huge car container
0:16:28 > 0:16:30that's come all the way from Spain.
0:16:30 > 0:16:33She's quite a big ship, so she's probably about 35-40,000 tonnes.
0:16:33 > 0:16:35Big ship, 160-odd metres long.
0:16:35 > 0:16:38- You're not going to sail that through here?!- Oh, yes!
0:16:40 > 0:16:42We have to drop nearly ten metres in the lock -
0:16:42 > 0:16:45that's the same as a three-storey building -
0:16:45 > 0:16:48to get to the same water level as out in the estuary.
0:16:48 > 0:16:50But once we're out there,
0:16:50 > 0:16:53it's obvious that the tide is rising rapidly.
0:16:55 > 0:16:57Wow, the estuary really is alive.
0:16:57 > 0:17:01You can see the water just rushing its way up the river.
0:17:01 > 0:17:04Look at the buoy over there, it's being bent towards inland.
0:17:04 > 0:17:07That's indicating that thousands and thousands of tonnes of water
0:17:07 > 0:17:11are working their way now directly up towards the port.
0:17:13 > 0:17:17The estuary is filling up at an amazing three metres per hour
0:17:17 > 0:17:20and, right on cue, our ship comes around the headland.
0:17:20 > 0:17:22SHIP FOGHORN BLOWS
0:17:22 > 0:17:25Dougal, there she is. That's your first sight of her.
0:17:25 > 0:17:26There she blows!
0:17:28 > 0:17:31But getting her into dock won't be easy.
0:17:31 > 0:17:34The fast-moving water heading up the estuary
0:17:34 > 0:17:37is trying to push the ship past the entrance to the port.
0:17:38 > 0:17:40This is the really critical moment.
0:17:41 > 0:17:44And this chunk of metal is being safely guided through
0:17:44 > 0:17:49- that very small gap, essentially using the tides.- That's right.
0:17:49 > 0:17:52Without the tidal range we have here, she would not get in there.
0:17:56 > 0:18:00One more ship has safely run the assault course
0:18:00 > 0:18:01of a fast-rising tide.
0:18:05 > 0:18:09But it's not just boats who have to battle the tides in this estuary.
0:18:09 > 0:18:12People regularly get caught out, too.
0:18:12 > 0:18:17Like here at Sully Island, which lies 200m off the South Wales coast.
0:18:17 > 0:18:21At low tide, it's possible to walk over to it.
0:18:21 > 0:18:23But in the last five years,
0:18:23 > 0:18:27more than 50 people have been cut off by the lightning-quick tides,
0:18:27 > 0:18:32keeping the local RNLI team, like Nicola, very busy.
0:18:33 > 0:18:36What we're stood on here is the causeway
0:18:36 > 0:18:38and this is the area that gets cut off by the tide.
0:18:38 > 0:18:41When the tide comes in, it covers this area so you couldn't cross.
0:18:41 > 0:18:42In just a couple of hours' time
0:18:42 > 0:18:45there will be six metres of water above us,
0:18:45 > 0:18:47so a very, very dangerous area.
0:18:47 > 0:18:51Nicola wants to show me exactly how quick the water arrives
0:18:51 > 0:18:55and why YOU should never ignore warnings about tides.
0:18:55 > 0:18:58'I think I'm going to get my feet wet,
0:18:58 > 0:19:02'so she's kindly kitted me out in a warm dry suit.'
0:19:04 > 0:19:08- Wow, it is surrounding us quite quickly, isn't it?- It is.
0:19:08 > 0:19:11- How much time do we have?- About ten minutes before we're covered here.
0:19:16 > 0:19:19The plan is to see how long I can keep my footing
0:19:19 > 0:19:21as the water rushes to fill the channel
0:19:21 > 0:19:23between the island and the mainland.
0:19:23 > 0:19:25The guys are on standby.
0:19:25 > 0:19:27So, bring on the tide!
0:19:27 > 0:19:30I've got a highly-trained lifeboat crew watching me,
0:19:30 > 0:19:34so this is definitely not something you should ever try.
0:19:34 > 0:19:36I've only been here a couple of minutes,
0:19:36 > 0:19:40and I can't believe how fast the water's already surrounding me.
0:19:40 > 0:19:44I'm really glad I've got my wellies on. Look at this!
0:19:44 > 0:19:48This is incredible! This water is rushing up the channel so quickly
0:19:48 > 0:19:52that the sea level is rising by 20cm every five minutes.
0:19:53 > 0:19:56I'm experiencing one of the world's fastest tides.
0:19:58 > 0:20:00It's getting deeper,
0:20:00 > 0:20:04bashing against my legs and you've got the wind to contend with, too.
0:20:04 > 0:20:08I'm keeping calm, because I've got the safety guys on hand.
0:20:08 > 0:20:12Five minutes in, and I'm struggling to keep my footing
0:20:12 > 0:20:16on the slippy rocks. The water's almost at the top of my legs.
0:20:17 > 0:20:20It's starting to get a little bit scary, actually.
0:20:20 > 0:20:23OK, this is all becoming a bit of a struggle,
0:20:23 > 0:20:26so it's time for the guys to come and get me out.
0:20:27 > 0:20:28I'm ready!
0:20:36 > 0:20:40'It's a big relief to be pulled to safety.'
0:20:40 > 0:20:42Thanks, guys!
0:20:42 > 0:20:43Whoa! Whoa, whoa, whoa!
0:20:43 > 0:20:47'It was very scary being cut off like that.
0:20:47 > 0:20:50'If you go to the beach, remember to make sure an adult knows
0:20:50 > 0:20:52'when the tides are coming in.'
0:20:56 > 0:20:59I really wasn't expecting it to be that powerful.
0:20:59 > 0:21:02A few minutes ago, it was trickling around my feet,
0:21:02 > 0:21:03and then, before you know it,
0:21:03 > 0:21:06two, three minutes later, it's up to your waist
0:21:06 > 0:21:09and it's choppy and it's powerful and you've got the wind.
0:21:12 > 0:21:15Without these guys, I would've been in a lot of trouble.
0:21:20 > 0:21:23The work done by life-savers like the RNLI
0:21:23 > 0:21:25keeps people out of harm's way at the seaside.
0:21:27 > 0:21:29It's no different back in California,
0:21:29 > 0:21:33where staying safe in the ocean is priority number one.
0:21:35 > 0:21:38The hundreds of lifeguards who work here
0:21:38 > 0:21:41are trained to be the best in the business.
0:21:41 > 0:21:45Today's the annual try-out for Santa Cruz lifeguards.
0:21:45 > 0:21:47These guys have to prove their swimming
0:21:47 > 0:21:50and life-saving skills are up to the job.
0:21:50 > 0:21:51That's because out in these oceans,
0:21:51 > 0:21:54there's a hidden menace known as rip currents,
0:21:54 > 0:21:56strong currents that pull people out to sea
0:21:56 > 0:21:59responsible for 80% of all the US lifeguards' rescues.
0:22:01 > 0:22:04Part of the team here is Gigi.
0:22:04 > 0:22:07She's been patrolling these beaches for eight years,
0:22:07 > 0:22:10so knows exactly how to spot a lurking rip current.
0:22:12 > 0:22:15She's taking me to a notorious beach,
0:22:15 > 0:22:18where swimmers and surfers face their threat every day.
0:22:21 > 0:22:24My job as a lifeguard is to be constantly looking to see
0:22:24 > 0:22:26if rip currents are being formed,
0:22:26 > 0:22:28because that is one of the number-one rescues on the beach.
0:22:28 > 0:22:32Rip currents can form on any sandy beach.
0:22:33 > 0:22:36Powerful waves pound the shoreline...
0:22:40 > 0:22:43..they break, releasing huge amounts of energy
0:22:43 > 0:22:46but then all that water wants to flood back out to sea.
0:22:49 > 0:22:52Sometimes a narrow channel forms in the sandy sea bed,
0:22:52 > 0:22:54and this is the route that the water uses
0:22:54 > 0:22:57to surge back out to sea in a rip current.
0:22:57 > 0:23:01So a fast-moving current is actually flowing AWAY from the beach.
0:23:03 > 0:23:05And when a swimmer gets caught in one,
0:23:05 > 0:23:07they won't realise it until they look back at the beach
0:23:07 > 0:23:10and see how far they've travelled out to sea.
0:23:10 > 0:23:12It doesn't necessarily suck you under,
0:23:12 > 0:23:15- it more sucks you straight out. - And that can be pretty dangerous?
0:23:15 > 0:23:17It can be extremely dangerous,
0:23:17 > 0:23:20and all your energy will force you to get tired
0:23:20 > 0:23:22and that's how a lot of drownings are caused.
0:23:23 > 0:23:25It all sounds pretty scary.
0:23:25 > 0:23:29But for my extreme challenge today, Gigi's got a plan to show me
0:23:29 > 0:23:32how helpless it feels to be trapped in a powerful rip current.
0:23:36 > 0:23:39It's too dangerous to swim in one for real,
0:23:39 > 0:23:40but out here in the bay,
0:23:40 > 0:23:43the conditions are perfect for our experiment.
0:23:45 > 0:23:48Well, what we're going to do is put your bungee cord
0:23:48 > 0:23:51around your waist and while the boat drifts,
0:23:51 > 0:23:54you'll be pulling, as if you're being pulled in the current.
0:23:56 > 0:23:58My challenge is to swim the 200 metres to shore,
0:23:58 > 0:24:01that's eight lengths of an ordinary swimming pool,
0:24:01 > 0:24:04with an elastic cord tying me to our boat.
0:24:06 > 0:24:08The extra energy I'll need to tow the boat behind me
0:24:08 > 0:24:12is going to feel just like swimming against a rip current.
0:24:12 > 0:24:14It's going to be very, very tough.
0:24:14 > 0:24:16And I hate cold water.
0:24:24 > 0:24:26OK, I'm going in, Gigi.
0:24:26 > 0:24:29- All right, Leo, you can do it. - Oh, looks cold!
0:24:29 > 0:24:32- Remember to stay calm.- Ahh, it's freezing!- Don't panic!- Oohhh!
0:24:35 > 0:24:38Although the sea is calm, this is a dangerous swim.
0:24:38 > 0:24:40Pulling the weight of the boat behind me
0:24:40 > 0:24:42is going to tire me out very quickly,
0:24:42 > 0:24:44so Gigi will be my own personal lifeguard
0:24:44 > 0:24:46alongside on her rescue board.
0:24:56 > 0:24:57I make a good start.
0:24:58 > 0:25:01Normally, I could swim 200 metres fairly easily,
0:25:01 > 0:25:02but as the elastic extends,
0:25:02 > 0:25:05I start to feel the pull of the boat behind me.
0:25:10 > 0:25:12Feels like I'm going nowhere!
0:25:13 > 0:25:16'I'm starting to tire and lose my technique,
0:25:16 > 0:25:19'just like being in a real rip current.'
0:25:21 > 0:25:23Keep going, Leo, you're doing good!
0:25:24 > 0:25:26The best way to escape if this was real
0:25:26 > 0:25:30would be to swim at right angles to the shore, like this...
0:25:30 > 0:25:32But I don't have that choice today.
0:25:35 > 0:25:36After just one and a half minutes,
0:25:36 > 0:25:40I've only travelled 20 metres but I'm exhausted.
0:25:41 > 0:25:44Fighting against the bungee has sapped all my energy.
0:25:47 > 0:25:50So, Gigi comes to the rescue.
0:25:57 > 0:25:58I feel like a drowned rat...!
0:26:03 > 0:26:05If that had been a real rip,
0:26:05 > 0:26:07I'd never have made it back to the shore.
0:26:07 > 0:26:10Thank goodness Gigi was here.
0:26:10 > 0:26:12These things are dangerous.
0:26:12 > 0:26:15He did good. He gave a good effort, he tried hard.
0:26:15 > 0:26:18It just goes to show that swimming straight into shore
0:26:18 > 0:26:21is not an easy way to exit a rip current.
0:26:23 > 0:26:25This was always going to be a tough challenge.
0:26:25 > 0:26:28I've only managed to swim 20 metres,
0:26:28 > 0:26:30a fraction of the distance to shore.
0:26:30 > 0:26:34Fighting against a rip current is a battle you're not going to win.
0:26:39 > 0:26:41The Fierce Earth team have been up close
0:26:41 > 0:26:43with the living, breathing ocean.
0:26:45 > 0:26:47The power of waves can be fun,
0:26:47 > 0:26:50but you must never turn your back on them.
0:26:51 > 0:26:54Always be aware of hidden rip currents.
0:26:56 > 0:26:59Tides creep up and down under the pull of gravity.
0:26:59 > 0:27:02Their power can helps ships into port,
0:27:02 > 0:27:06but if you don't stay alert, their rapid rise can catch you out.
0:27:07 > 0:27:10It's getting deeper, bashing against my legs.
0:27:10 > 0:27:12Where the land meets the sea...
0:27:14 > 0:27:17..we've learned the Earth can get very fierce.
0:27:20 > 0:27:23In the very special next episode of Fierce Earth...
0:27:23 > 0:27:26Leo chooses his most extreme moments ever.
0:27:26 > 0:27:29Plus, an all-new exclusive challenge.
0:27:29 > 0:27:32I'm going to take to the skies and fly like a bird.
0:27:32 > 0:27:34He's saving the fiercest till last.