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On today's Fierce Earth, it's all about the awesome power of the sea.
Here, in Santa Cruz, California, home of monster waves.
And we have monsters here in the UK - monster tides.
We'll find out how the sea goes from this...
..To this. It's incredible!
It's wet suits on for the Fierce Earth team, as we hit the surf
to unlock the secrets of king-sized waves.
Leah experiences one of the world's fastest rising tides.
It's starting to get a little bit scary, actually.
Remember to stay calm.
I meet the awesome power of a rip current head on.
-Good job, Leo, keep it up!
And Captain Dougal rides the tides to guide a massive ship into port.
That's one incredible sight to see.
What happens when the ground shakes,
the seas rise up and the air tears itself apart?
The Fierce Earth team move in,
taking on the most powerful forces on the planet.
Get ready for Fierce Earth -
the Earth, and how to survive it.
The world's shorelines are in a never-ending battle with the sea.
Battered by the huge force of the waves,
year in year out, 24 hours a day.
In the UK, the storms of Christmas 2013
changed the shape of the coast for ever.
Around the planet more than 3 billion people live by the sea.
Waves can be dangerous and even cost lives,
but when the sea is safe, they can be a lot of fun.
We're here in Santa Cruz, California,
home to some of the most famous coastline in the world.
As you can see, it's pretty popular with surfers -
that's because the waves here can be absolutely massive.
We're here to find out why.
People come from all over the world to ride these waves,
which can be up to 15 metres tall -
that's as high as a five-storey building.
This place is paradise for thrill-seekers -
making this beach one of the best surfing locations in the world.
To understand how waves bring the sea to life,
we first need to know what one is, and how it moves.
OK, let it go on the ground...
Whenever we think of waves, we think of the sea,
but, in fact, waves are all around us.
I can create my own with this skipping rope. Watch.
Notice when Leo flicks his wrist,
the energy transfers along the rope in the form of a wave.
And that's exactly what happens when waves move through the sea.
Waves are energy passing through water,
and sometimes travelling for thousands of miles.
The next piece of land in that direction is Japan,
more than 5,000 miles away.
Some of these waves have travelled almost that far.
That distance is important in creating the huge waves
that crash against this coast.
If you look out there,
you can just see Clare bobbing up and down on a boat.
Hi, Leo! Although it looks like the water itself
is moving towards Leo on the shore,
a wave is actually ENERGY passing through the water.
You can see the boat bobbing up and down
as the energy of the wave passes underneath it,
then make its way to shore where it breaks.
OK, Clare, you can come in!
So, where does this energy come from?
Remember the skipping rope?
Leo started that wave with a flick of his wrist.
Well, out at sea, waves start with wind.
Imagine this swimming pool is the Pacific Ocean.
We're going to use a leaf blower to create a storm with high winds.
Now, let's send Leo over to the other side of the ocean.
OK, Clare, I'm in California.
Now it's the fun part.
I'm going to start a mini storm and send it in Leo's direction.
You can see how rough the seas are,
but as the waves move away from the storm,
they become organised lines moving across the ocean
towards the shoreline.
You can see the energy from the storm
that Clare has created over there
has transferred across the pool in the form of these waves.
So, it's wind that fuels waves.
When a storm whips up out in the ocean,
the longer and harder it blows,
the more energy it propels towards the shore.
A wave's long journey finally ends near the coast when it breaks,
but this can only happen
when the final piece of the jigsaw falls into place.
As the ocean gets shallower near to the shore,
the wave slows down due to the friction of the sea bed.
As the water begins to mound up,
the wave takes on its distinctive shape.
Once it gets too tall, it breaks and rolls up the beach.
At Santa Cruz, California, all these ingredients come together perfectly.
Thousands of miles of ocean to build up all that energy,
and then the perfect sea bed
to create monster breakers like these.
It's no wonder one sport has made its home here.
Leo and I are here to have a go, and we've got two very special teachers.
Sam, 12, and Ben, 15, have lived next to the ocean all their lives.
They've been surfing since they were five and are now Junior Champions.
They're pretty good.
Their challenge today is to get Clare and I doing this...
-Nice to meet you, Ben.
-Hi, Sam, how you doing?
-So, you're going to teach us how to surf?
-Yeah, it should be fun.
-So, do you guys surf every day?
And what makes it so fun
that you can surf on the coldest day in winter?
You just get good waves
and it gets you happy when you do good turns.
By following the surf forecasts online,
the boys know in advance what the sea conditions are going to be like.
But sometimes this sea gets dangerous,
and that's where years of experience come in.
It's important to understand where to be,
so you don't get caught in a dangerous situation.
And also you just never want to turn your back to the ocean
because you'll never know if, like, a big wave is going to take you out.
-Trying to read the waves.
-Your board doesn't want to be
between you and the wave or, like, it's going to hit you.
Today, the sea is fairly calm, but the waves off this coast
are capable of smashing boats and surfboards in two
with their awesome force.
There's no way you'd get me out in a sea like that!
If you want to harness the energy of a wave on a surfboard,
you have to paddle yourself up to speed
and leap from a lying position to standing up.
Sam and Ben make it look very easy.
Something tells me it isn't...
So, if you're laying down on the board like this...paddling...
whatever foot you put forward...
I don't think I could even do that.
Let's have a go.
I don't think we're going to be the next World Champions!
Too far forward, your nose is going to dig into the water
-and you'll face-plant.
-Woo-hoo! Going to be fun!
You're getting there, but you're going to need a lot of practice
-before you get in the ocean.
-'I think he may be right.'
OK, nice one.
But, somehow, we persuade them to let us loose in the ocean.
We head off to a beginners' beach where the boys learned to surf.
Conditions are perfect, waves of about 1.5 metres.
Easy for Sam and Ben,
but they look like skyscrapers to us as we paddle out.
After a few failed attempts to catch a wave, we're going nowhere.
But, eventually, with a friendly shove from Ben,
I get up on the wave.
The energy pushes me hundreds of metres into the shore,
it's an amazing feeling.
Now, it's Leo's turn, and he has to go one better and stand up.
-You guys were amazing.
Without you two I just wouldn't have even got in the water.
Yeah, you did really good. That one, you got in it.
Sam, you're amazing! 12-year-old kid teaching me how to surf.
-Got it first time!
I can't believe the immense power of those waves.
It was just incredible.
And to think that thing travelled all the way across the ocean
and just for that split second, you get to ride that force.
-It's amazing, isn't it?
-It's better than that paddling!
I want to go and get another one!
Still to come, Leo sees if he has what it takes to beat
the ocean's hidden menace - the rip current.
Feels like I'm going nowhere!
Oceans cover 71% of the Earth's surface
and contain 97% of the planet's water.
The energy carried by waves means lots of fun for surfers
and, as we now know, waves don't actually move the water.
Think about the last time you went to the seaside.
Throughout the day, the water level changes
causing the beach to shrink and grow.
This is the tide coming in and out.
6,000 miles away from Santa Cruz in California is the UK,
a place where tides are a very big deal.
We're in South Wales.
Behind us is the Severn Estuary,
the mouth of Britain's longest river.
The harbour here at the moment is completely full of water.
But if we were just to wait another six hours,
the sea level's going to drop an incredible 12 metres.
To this. It's incredible.
All of this variation in sea level is due to tides.
And we're here to find out how they work.
Tides are one of the great forces on our planet.
Every day, most coastlines
experience two high tides and two low tides.
The driving force behind this constant change is gravity.
What's going to happen if I throw this ball into the sky?
Is it going to stay there, is it going to go upwards?
No, we know what's going to happen.
It's going to fall to the ground.
That's because it's being pulled by a force called gravity
drawing us all into the centre of the Earth.
It keeps me standing here, it's dropped the ball to the ground
and it also keeps the oceans in place.
But we also know the oceans don't stay exactly where they are.
They are constantly on the move,
coming in and going out with the motion of the tides.
Now, why is that?
If you imagine this is the Earth with its own gravitational force,
there's something missing here
and that's up there in the sky.
The moon travels round the Earth in one month
and the moon, as a planet, has its own gravitational force
but it's much weaker than that of the bigger Earth,
but it's still strong enough to affect the oceans.
As the moon moves around the Earth throughout each month,
it pulls the oceans slightly towards it, helping to cause the tides.
But there's another thing to think about.
Imagine this is us on the Earth.
The Earth is constantly spinning,
giving us one day and one night every 24 hours.
So, every 24 hours the Earth moves twice
through the deep water giving us high tides,
and twice through the shallow water giving us low tides.
The difference between the height of a high tide
and a low tide is called a tidal range.
Almost nowhere in the world sees a bigger tidal range
than here in the Severn Estuary.
That's because of its distinctive funnel shape.
If you've got Wales here and England there,
the tide is being forced up this funnel-shape river mouth, like so.
There's only one thing going to happen.
The water mass is moving in and in as the tide rises,
but it's getting funnelled in and there's land either side -
it can't go anywhere,
so it builds up, just like this sand here,
and builds up and builds up and builds up.
So, around about this part of the estuary,
that's where you get the highest tides,
because it's being forced into this funnel shape.
This massive tidal range can be used to great effect by ports,
because it means, at high tide,
you can get enormous ships in and out of docks,
like this one in Bristol.
But you don't have long to do it.
Every day, thousands of tonnes of cargo
come in and out of this massive dock.
They can bring anything from cars,
coal, even toys, to Britain.
But getting the huge ships that carry all of this
in and out of the docks is a real assault course because of the tides.
By measuring the distance of the moon from the Earth
and understanding the shape of the sea bed,
scientists can predict for many years ahead
when we will have high and low tides.
This channel controls the shipping in and out of the dock.
Now, the level of water in the dock always stays high
because of these giant gates, they lock the water back.
But at low tide, we've got a problem.
Look at the other side here.
You can see the water level's down to less than a metre,
you can see mud banks over there.
There's no way we're going to get a large ship into the docks
when the tide is low.
There's only one way to do that -
wait until the tide has risen high enough
and very carefully guide the ship in.
I'm here to meet Jerry.
-Nice to see you. Welcome aboard.
He's like an air-traffic controller for ships.
His job is to get as many ships into port as possible
during the small window of opportunity.
He needs to know exactly when the tide will be high
and when it will be low.
We're heading out into the estuary to meet a huge car container
that's come all the way from Spain.
She's quite a big ship, so she's probably about 35-40,000 tonnes.
Big ship, 160-odd metres long.
-You're not going to sail that through here?!
We have to drop nearly ten metres in the lock -
that's the same as a three-storey building -
to get to the same water level as out in the estuary.
But once we're out there,
it's obvious that the tide is rising rapidly.
Wow, the estuary really is alive.
You can see the water just rushing its way up the river.
Look at the buoy over there, it's being bent towards inland.
That's indicating that thousands and thousands of tonnes of water
are working their way now directly up towards the port.
The estuary is filling up at an amazing three metres per hour
and, right on cue, our ship comes around the headland.
SHIP FOGHORN BLOWS
Dougal, there she is. That's your first sight of her.
There she blows!
But getting her into dock won't be easy.
The fast-moving water heading up the estuary
is trying to push the ship past the entrance to the port.
This is the really critical moment.
And this chunk of metal is being safely guided through
-that very small gap, essentially using the tides.
Without the tidal range we have here, she would not get in there.
One more ship has safely run the assault course
of a fast-rising tide.
But it's not just boats who have to battle the tides in this estuary.
People regularly get caught out, too.
Like here at Sully Island, which lies 200m off the South Wales coast.
At low tide, it's possible to walk over to it.
But in the last five years,
more than 50 people have been cut off by the lightning-quick tides,
keeping the local RNLI team, like Nicola, very busy.
What we're stood on here is the causeway
and this is the area that gets cut off by the tide.
When the tide comes in, it covers this area so you couldn't cross.
In just a couple of hours' time
there will be six metres of water above us,
so a very, very dangerous area.
Nicola wants to show me exactly how quick the water arrives
and why YOU should never ignore warnings about tides.
'I think I'm going to get my feet wet,
'so she's kindly kitted me out in a warm dry suit.'
-Wow, it is surrounding us quite quickly, isn't it?
-How much time do we have?
-About ten minutes before we're covered here.
The plan is to see how long I can keep my footing
as the water rushes to fill the channel
between the island and the mainland.
The guys are on standby.
So, bring on the tide!
I've got a highly-trained lifeboat crew watching me,
so this is definitely not something you should ever try.
I've only been here a couple of minutes,
and I can't believe how fast the water's already surrounding me.
I'm really glad I've got my wellies on. Look at this!
This is incredible! This water is rushing up the channel so quickly
that the sea level is rising by 20cm every five minutes.
I'm experiencing one of the world's fastest tides.
It's getting deeper,
bashing against my legs and you've got the wind to contend with, too.
I'm keeping calm, because I've got the safety guys on hand.
Five minutes in, and I'm struggling to keep my footing
on the slippy rocks. The water's almost at the top of my legs.
It's starting to get a little bit scary, actually.
OK, this is all becoming a bit of a struggle,
so it's time for the guys to come and get me out.
'It's a big relief to be pulled to safety.'
Whoa! Whoa, whoa, whoa!
'It was very scary being cut off like that.
'If you go to the beach, remember to make sure an adult knows
'when the tides are coming in.'
I really wasn't expecting it to be that powerful.
A few minutes ago, it was trickling around my feet,
and then, before you know it,
two, three minutes later, it's up to your waist
and it's choppy and it's powerful and you've got the wind.
Without these guys, I would've been in a lot of trouble.
The work done by life-savers like the RNLI
keeps people out of harm's way at the seaside.
It's no different back in California,
where staying safe in the ocean is priority number one.
The hundreds of lifeguards who work here
are trained to be the best in the business.
Today's the annual try-out for Santa Cruz lifeguards.
These guys have to prove their swimming
and life-saving skills are up to the job.
That's because out in these oceans,
there's a hidden menace known as rip currents,
strong currents that pull people out to sea
responsible for 80% of all the US lifeguards' rescues.
Part of the team here is Gigi.
She's been patrolling these beaches for eight years,
so knows exactly how to spot a lurking rip current.
She's taking me to a notorious beach,
where swimmers and surfers face their threat every day.
My job as a lifeguard is to be constantly looking to see
if rip currents are being formed,
because that is one of the number-one rescues on the beach.
Rip currents can form on any sandy beach.
Powerful waves pound the shoreline...
..they break, releasing huge amounts of energy
but then all that water wants to flood back out to sea.
Sometimes a narrow channel forms in the sandy sea bed,
and this is the route that the water uses
to surge back out to sea in a rip current.
So a fast-moving current is actually flowing AWAY from the beach.
And when a swimmer gets caught in one,
they won't realise it until they look back at the beach
and see how far they've travelled out to sea.
It doesn't necessarily suck you under,
-it more sucks you straight out.
-And that can be pretty dangerous?
It can be extremely dangerous,
and all your energy will force you to get tired
and that's how a lot of drownings are caused.
It all sounds pretty scary.
But for my extreme challenge today, Gigi's got a plan to show me
how helpless it feels to be trapped in a powerful rip current.
It's too dangerous to swim in one for real,
but out here in the bay,
the conditions are perfect for our experiment.
Well, what we're going to do is put your bungee cord
around your waist and while the boat drifts,
you'll be pulling, as if you're being pulled in the current.
My challenge is to swim the 200 metres to shore,
that's eight lengths of an ordinary swimming pool,
with an elastic cord tying me to our boat.
The extra energy I'll need to tow the boat behind me
is going to feel just like swimming against a rip current.
It's going to be very, very tough.
And I hate cold water.
OK, I'm going in, Gigi.
-All right, Leo, you can do it.
-Oh, looks cold!
-Remember to stay calm.
-Ahh, it's freezing!
Although the sea is calm, this is a dangerous swim.
Pulling the weight of the boat behind me
is going to tire me out very quickly,
so Gigi will be my own personal lifeguard
alongside on her rescue board.
I make a good start.
Normally, I could swim 200 metres fairly easily,
but as the elastic extends,
I start to feel the pull of the boat behind me.
Feels like I'm going nowhere!
'I'm starting to tire and lose my technique,
'just like being in a real rip current.'
Keep going, Leo, you're doing good!
The best way to escape if this was real
would be to swim at right angles to the shore, like this...
But I don't have that choice today.
After just one and a half minutes,
I've only travelled 20 metres but I'm exhausted.
Fighting against the bungee has sapped all my energy.
So, Gigi comes to the rescue.
I feel like a drowned rat...!
If that had been a real rip,
I'd never have made it back to the shore.
Thank goodness Gigi was here.
These things are dangerous.
He did good. He gave a good effort, he tried hard.
It just goes to show that swimming straight into shore
is not an easy way to exit a rip current.
This was always going to be a tough challenge.
I've only managed to swim 20 metres,
a fraction of the distance to shore.
Fighting against a rip current is a battle you're not going to win.
The Fierce Earth team have been up close
with the living, breathing ocean.
The power of waves can be fun,
but you must never turn your back on them.
Always be aware of hidden rip currents.
Tides creep up and down under the pull of gravity.
Their power can helps ships into port,
but if you don't stay alert, their rapid rise can catch you out.
It's getting deeper, bashing against my legs.
Where the land meets the sea...
..we've learned the Earth can get very fierce.
In the very special next episode of Fierce Earth...
Leo chooses his most extreme moments ever.
Plus, an all-new exclusive challenge.
I'm going to take to the skies and fly like a bird.
He's saving the fiercest till last.