Original subtitles

Narrator: Untameable and mighty.

The oceans are our last true wilderness.

They cover 70% of the surface of our planet.

Their power can fill us with awe.

And, at times, fear.

But, today, we are also beginning to reveal their hidden wonders.

Off the wild coast of South Africa,

bottlenose dolphins face some of the roughest seas on earth.

But for them, big waves are an opportunity for play.

And why do they do it?

To build friendships

and strengthen family bonds.

And, also, for the sheer joy of it.

There is so much more to discover about our oceans

and their importance to us.

Over 40% of us live within 609 miles of the seas.

But it is still the least known part of our planet.

Today, scientists and film makers

are heading out to explore the seven seas.

Equipped with the latest technology,

their mission is to bring us a new understanding

of life beneath the waves.

At a time when the health of our oceans is increasingly under threat,

this has never been more urgent.

Our journey begins in the warm, clear shallows of the tropics.

Home to coral reefs.

They occupy less than 1% of the ocean floor.

Yet, they are home to a quarter of all marine species.

Competition is fierce in these crowded underwater cities.

We are learning just how noisy they are

with state-of-the-art underwater microphones.

Amazingly, fish can talk to each other.

But sometimes it's better to stay quiet.

On Australia's great barrier reef,

one character is challenging our understanding of fish intelligence.

This is a tusk fish.

He does something few would have believed a fish could do.

Every morning, he makes a journey out to the edge of the reef.

He is looking for breakfast.

A clam.

But how to crack it open and get to the meat inside?

He carries it all the way back to his special kitchen.

A bowl shaped coral.

With a particular bump inside that he always uses.

It's not easy

if you have no hands.

Oops.

Dropped it again.

But he's got great determination.

At last.

So here's a fish that uses a tool.

Some fish are much cleverer than we ever thought.

Even ocean creatures that we already knew where smart

are continuing to surprise us.

This bottlenose dolphin mum

is about to teach her calf an important lesson

here in the red seas.

She leads him to a particular bush-like coral

called a gorgonian.

The family rub themselves through the fronds.

The calf is watching

and learning.

Gorgonian fronds are covered with a mucous

that can have anti-inflammatory and anti-microbial properties.

So, maybe, the dolphins are doing this to protect themselves from infection.

The calf may be too young to join in,

but his family's secret knowledge of the coral reef

is teaching us to search for new medicines here, too.

As we leave the tropics and head into cooler waters,

we enter the temperate seas.

These are home to mysterious undersea forests.

Giant kelp, a seaweed,

towers 200 feet high,

growing two feet a day under the summer sunshine.

Marine plants are the lungs of our planet.

They pump out as much oxygen

as all the forests and grassy plains on land.

Within their tangled undergrowth, extraordinary discoveries await.

In the shark-filled kelp forests of Southern Africa

lives one brave little octopus.

Pyjama sharks are every octopus' worst nightmare.

But the octopus has a trick up her sleeve.

In a behaviour previously unknown to science,

she disguises herself with shells,

creating a suit of armour.

The shark can sense its prey.

But the shells protect her.

Superior wits allow this octopus to stay alive.

But just as we're getting to know these forests,

we're recognising their vulnerability.

Off north America's pacific coast, great stands of kelp

are being felled

by armies of ravenous sea urchins.

They much through the roots with razor-sharp teeth.

But some kelp forests here have unlikely guardians.

Seg otters.

Because they live their entire lives in such cold waters,

they need to eat

30% of their body weight a day to stay warm.

One of their favourite foods is sea urchin.

By removing the urchins,

the otters allow the forests to flourish.

And with all this food, they're having a baby boom.

Now, in a few remote areas,

sea otters are creating vast rafts

in numbers not seen for more than a century.

As we leave our coasts and head for the high seas,

we enter the open ocean.

Covering over half our planet's surface,

it's the world's greatest wilderness.

A vast empty void

where there's nowhere to hide and little to eat.

And, yet, it's home to vast numbers of dolphins.

Spinner dolphins.

They sweep this marine desert for food

in a superpod 5,000 strong.

And they're leading this research vessel to a rare feeding event.

But to find it, they have to be fast.

Using echolocation, they lock on to their prey.

Great shoals of lantern fish.

By pinning the school against the surface,

the dolphins keep this fleeting opportunity alive.

But all this food attracts other predators.

Yellowfin tuna.

They rip through the lantern fish

in a whirling carousel at 40 miles an hour.

Now, the sea begins to boil.

Finally, mobula rays

with ten foot wing spans swoop in.

In just 15 minutes, all that's left is a silvery confetti of scales.

These fleeting events are becoming rarer

as we continue to overfish our high seas.

The open ocean may be featureless

but isolated volcanic peaks rise abruptly from deep water...

Hinting at a secret world below.

The deep ocean is as challenging to explore as space.

We know more about the surface of Mars than we do about the deep sea.

Now, from the research vessel alucia,

we can dive these uncharted depths

to discover our final frontier.

As we descend, the pressure increases relentlessly.

Six hundred feet down, we enter an alien world.

The twilight zone.

A pyrosome.

A tube of jelly six-feet long.

And, stranger still barrel eye.

A fish with a transparent head,

so that it can look up through its skull.

Eventually, we reach the deep sea floor.

A layer of mud. In places, a mile thick.

Over time, the mud here slowly decays.

Creating volcanoes of methane gas.

The deep maybe hostile,

but it's also home to the weird and wonderful.

Dancing crabs called yetis.

Because of their hairy arms

on which they farm bacteria to eat.

A dumbo octopus.

With ear-like-fins to hover above the muddy sea floor.

There are also corals here.

With more species than on shallow tropical reefs.

Astonishingly, we're now finding there's more life down here

than anywhere else on earth.

This may seem an alien world,

but we are more closely connected to the deep

than we ever thought possible.

Thanks to great ocean currents.

These begin at the frozen poles.

Here in Antarctica,

the surface waters are so cold and heavy

that they sink.

That creates immense rivers of water that flow into the deep

where they power a global network of currents.

These currents flow from the frozen poles

to the warm tropics and back again,

linking every ocean.

They redistribute heat around the planet

maintaining a climate favourable for life on earth.

From producing the oxygen we breathe to controlling our weather,

the oceans are our life-support system.

But just as we're discovering how dependant we are on the oceans,

there are worrying signs that they are warming

at a faster rate than every before in human history.

And nowhere is this more extreme

than in the arctic.

Walrus prefer to rest on sea ice.

But with less ice than ever before,

hundreds of quarrelsome mothers now have to haul out on dry land.

It's far from an ideal nursery.

Nor is it safe from polar bears.

A full-grown male walrus is too big for a bear to tackle.

So, it's looking for a walrus baby.

This young mother

needs to find somewhere for her pup to rest.

The only safe places

are the last remaining pieces of floating ice.

The trouble is...

That they're slippery...

And some walrus pups are just too heavy.

The best icebergs are already full.

It only takes another one-ton mum

to tip the balance.

Finding a safe place on these melting shores

becomes harder and harder.

Solving these problems together helps create a bond so strong

that the mother and her youngster

will stay in contact for the rest of their lives.

Just as we're beginning to understand

the sophisticated lives of sea creatures,

so we begin to recognise the fragility of their home.

As we explore every part of the remote seas

and meet astonishing animals

We've begun to appreciate the importance of our oceans.

There has never been a more crucial time to continue this journey of discovery.

Because our future, too, depends on a healthy blue planet.

And who knows what other secrets are out there...

Waiting to be discovered.

Man 1: The first time I saw the tusk fish

cracking a clam open on its little anvil,

I was just gobsmacked.

He must have hit it well over 50 times.

We call him Percy. Percy the persistent.

Man 2: No one has previously dived to a thousand metres in Antarctica.

It is exciting. It is thrilling.

And, yet, it's also slightly terrifying.

Man 3: Control, control, deep rover.

My depth is 1,000 metres on bottom. Over.

Man 2: We can't explore the deep ocean remotely

without actually going there ourselves.

And we're the first to do that here.

Man 4: It's an ordeal to get it going

because it's a big, giant piece of equipment.

But we now have it. We're heading out to find some walrus.

We just had our first successful shoot with the megadome, and it was great.

I mean, it was that classic shot of an iceberg,

where six-sevenths of the berg is underwater

and then on top was two walrus.

Man 5: I'm feeling pretty nervous. The erm...

It's a big swell out there.

This will probably be the biggest seas, I think I've ever been out in.

The wave actually hit the back of the sled.

Hit a big bump and almost bounced off trying to hold the camera,

and just rode out of there.

Just another day at the office.