Original subtitles

The world's greatest wilderness,

the open ocean.

It covers over half the surface of our planet.

Here, there is nowhere to hide

and little to eat.

It's the marine equivalent of a desert.

And patrolling this desert, spinner dolphins.

They stick together.

In a super pod, 5,000-strong.

That maximises their chances of finding something to eat.

Like all who live here,

they must go to extraordinary lengths

to make their home in the big blue.

There are rare moments

when these empty seas can explode with life.

Lanternfish off the Pacific Coast of Costa Rica.

They're scarcely bigger than minnows.

But what they lack in size,

they make up for in numbers.

They are one of the most numerous fish anywhere.

Normally they only come to the surface at night

to feed on plankton.

But this immense shoal has risen during the day,

almost certainly in order to spawn.

For the dolphins, this would be a bonanza.

They have located the shoal using their echo-sounding calls.

But they have to get to it quickly.

They are not the only hunters here.

Yellowfin tuna have also detected the shoal.

And behind them, with their two-metre wingspans,

Mobula rays.

Now sailfish,

one of the fastest fish in the sea, have joined the chase.

The lanternfish may return to the deep at any moment.

But now, the dolphins have got here.

They swim beneath the shoal, pinning it to the surface

and forcing the lanternfish to pack more closely together.

And now the sea begins to boil.

The tuna charge into the shoal at over 40 miles an hour.

The slower swimming rays arrive at last.

With their immense mouths agape,

they scoop up the lanternfish by the hundred.

The shoal has now been largely dispersed,

and the sailfish pick off the survivors.

In just 15 minutes,

all that's left is a silvery confetti of scales.

But here, such feasts are only too infrequent.

Whilst the dolphins perform great feats of endurance,

others are driven to even greater extremes

to find food in this ocean desert.

A sleeping giant.

A sperm whale.

This family is resting between bouts of feeding.

Who knows what the owners

of the biggest brain in the planet dream about.

One has a calf

It's about two weeks old, but still dependent on its mother's milk.

It's hungry.

It communicates with its mother using a pattern of clicks.

But its mother slumbers on.

The calf, covered in suckerfish,

of which it can't yet rid itself,

has to be patient.

Sleep over, and refreshed,

the whales move on.

Sperm whales don't wait for their prey to rise to the surface.

They swim down into the depths to find it.

They take a series of heavy breaths...

...to saturate their blood with oxygen.

Then, down they go.

This entire family dives together in search of squid.

The mother will push her body to the limits of her endurance,

and already it's hard for her calf to keep up with her.

The calf sticks to its mother as closely as it can

touching her frequently

as if for reassurance.

But 300 metres down,

it seems the calf can't hold its breath any longer.

In their early years, calves are forced to sit out the hunt.

The adults continue their dive.

The mother changes her calls

into a series of louder and more rapid clicks.

She's now using sonar to hunt down shoals of squid.

At 800 metres, a burst of clicks.

Then, silence.

She's made a catch.

The hunters could be away foras much as an hour.

Finally, the mother returns from the deep

with a stomach full of squid.

After a long wait,

this hungry calf can take some milk.

It's one of the richest produced by any mammal.

And the calf guzzles a bathful of it a day.

It may be six years before a calf masters the art of deep diving

and is able to find food for itself

The emptiness of the big blue is what makes life so hard for hunters.

But it's this emptiness

that makes it comparatively safe for prey.

A baby turtle, hatched just days ago,

is leaving the crowded, dangerous waters of the coast,

and heading for the open ocean.

Only recently have we begun to solve the mystery

of where baby turtles disappear to in their early years.

To start with, they fill their little stomachs with plankton.

But soon, they need something more substantial.

Hundreds of miles offshore, in every ocean,

there are communities of young castaways.

So, anything that floats attracts them.

A log.

It may have been at sea for several years,

and it has already become the centre of a small community.

Young pufferfish are here for the same reason.

A floating log is just the kind of refuge

this young turtle has been looking for.

Here, there's not only seaweed on which to graze,

but barnacles.

But it's important to stay undercover.

A young ocean-going silky shark is here too.

It's learning what tastes good,

and what doesn't.

We now know that many young turtles

stay in such places for several years

until adulthood.

Even if it means facing the full force of the high seas.

The sun, beating down on the deep blue,

warms the surface waters so that they evaporate.

As the vapour rises, it condenses into clouds.

They rapidly build into gigantic burgeoning towers,

which eventually generate violent storms,

some a thousand miles across.

Hurricane force winds sweep across the open ocean,

building waves that could rise to 30 metres tall.

Out here, ships have been known to sink without trace.

One hundred and thirty million containers

are shipped across the oceans every year.

And on average, four of them fall into the sea every day.

In 1992,

a few were lost that contained a consignment of bath toys,

including 7,000 plastic ducks like these.

They started their travels a thousand miles off Alaska.

Some drifted right across the Pacific Ocean,

and reached Australia.

Others were carried north,

and landed on shores between Russia and Alaska.

They even found their way into the high Arctic.

One duck, having been at sea for 15years,

and crossing three oceans,

eventually landed on the west coast of Scotland.

Their travels vividly illustrate

how a network of currents connects all our oceans

into one gigantic circulatory system.

Many of the inhabitants of the big blue

rely on these currents to carry them to feeding grounds.

The blue shark.

It travels over 5,000 miles a year,

riding on the currents,

supported by its broad wing-shaped fins.

This one may not have eaten for two months.

But the currents can carry promising traces of fatty oils

from many miles away

and will lead it to its next meal.

After days of travel, the smell of food gets stronger.

A dead whale, recently struck by a ship.

This could be a real feast,

but the blue shark must be cautious.

Great white sharks,

ten times heavier than the blue,

are highly possessive around a whale carcass.

Great whites are eager to feed on energy-rich whale blubber,

which we now know forms a major part of their diet.

Once the great white has had its fill,

smaller sharks, like the blue shark,

tackle what's left of the carcass.

As the oils from this dead whale spread more widely,

more and more blue sharks appear.

Within days, the carcass will be stripped of its blubber.

Then, no longer kept buoyant by its oil,

it will sink into the depths below.

The blue, with its reserves of fat replenished,

can now survive for another two months without eating.

Over half of all animals in the open ocean

drift in currents.

Jellyfish cross entire oceans

feeding on whatever happens to tangle with their tentacles.

Some can grow to a metre, even two metres across.

And when, by lucky chance,

they encounter a patch of sea rich in plankton,

their numbers explode.

It's such a successful strategy,

that jellies are one of the most common lifeforms on the planet.

But among the jellies,

and looking somewhat like them,

is a rather more complex and sinister creature.

The Portuguese man o' war.

It floats with the help of a gas-filled bladder,

propped by a vertical membrane.

With that serving as a sail,

it maintains a steady course through the waves.

Long threads trail behind it,

some as much as 30 metres long.

Each is armed with many thousands of stinging cells.

A single tentacle could kill a fish,

or, in rare cases, a human.

But among its lethal tentacles,

lurks a man o' war fish

that feeds by nibbling them.

Whilst this fish has some resistance to the stings,

it must still be extremely careful.

Most other fish are not so lucky.

A tentacle has caught this one, and reels it in.

It's already paralysed.

Specialised muscular tentacles

transfer the victim to others that digest the catch,

liquefying it with powerful chemicals.

Eventually all that is left

is a scaly husk.

This voracious man o' war

may collect over a hundred small fish in a day.

For the most part, the big blue seems featureless.

A place where the winds blow, uninterrupted by land.

But beneath the surface,

there are long mountain ranges, deep trenches,

and isolated volcanic peaks

that make it far more varied than the human eye can see.

We're only just discovering in any detail

how the inhabitants of the big blue exploit that.

A lonely whale shark

on a specia journey.

She is as long as a small aircraft,

and she weighs over 20 tonnes.

Like many sharks, she does not lay eggs,

but gives birth to live young.

She carries up to 300 of them

in her swollen belly.

She may be the biggest fish in the sea,

but the place where whale sharks give birth has not yet been found.

Today, however, we may be a step closer to solving this mystery.

We have known that great numbers of whale sharks,

at certain times of the year,

appear around the Galápagos Islands.

Here they assemble around a tiny islet

that rises abruptly from particularly deep water.

It's known as Darwin Island.

Here, swirling currents bring out nutrients from the deep,

so enriching these waters

that they attract great concentrations of fish

from far and wide.

Thousands of hammerhead sharks also assemble here.

They are nearly all female.

They, too, it seems, have come here to breed.

The whale shark receives an extraordinary welcome.

Silky sharks, themselves three metres long,

bounce against her rough skin,

perhaps to scrape off any parasites they might have.

These sharks could be a danger to any newly-born young.

So, perhaps to avoid them,

the whale shark dives

down to around 600 metres.

And there, she may release her young.

In these great depths,

away from the predators that hunt in the waters above,

and with abundant food,

her babies could grow and eventually disperse.

No one, it is true,

has ever seen young ones in these little-visited depths.

But the fact that hundreds of expectant whale sharks

come here every year,

is strong evidence that somewhere here

lies the nursery of the biggest fish in the sea.

There are almost 30,000 sizeable islands

scattered across the world's oceans.

One of them is South Georgia.

An ideal place for those ocean-dwellers

who are compelled to land in order to breed.

The wandering albatross.

It may spend as much as a year continuously at sea,

searching for food,

gliding on wings that are three and a half metres across.

The biggest of any living bird.

The entire world population

of 16,000 wanderers nest on South Georgia

and half a dozen or so of the other smaller islands

that lie in the Southern Ocean.

It's spring.

And this bird is returning to the nest site it has always used.

Its lifelong partner is already here.

In South Georgia,

individual birds have been studied for their entire lives

revealing that older pairs in their late thirties

will go to extraordinary lengths

to give their young the best possible start in life.

This chick is now several weeks old,

but still has its warm, downy coat.

The chick will need a regular supply of regurgitated fish and squid.

With food so scarce in the open ocean,

both parents may have to scour thousands of square miles

Just to provide enough for one meal.

Ageing parents struggle on all through the Antarctic winter,

to raise a chick that is big, strong and healthy.

After some 130 days,

the youngster begins to replace its down with flight feathers.

Finally, nine months after their egg was laid,

this chick is ready to leave.

Of all the chicks they've reared in recent years,

such a favoured chick will have the best chance of survival.

But it will also be their last.

Elderly parents never recover from their exertions.

They will soon leave this island,

never to be seen again.

Surviving in the open ocean

has always tested animals to the limit.

But today, they face a new, additional threat.

Plastic.

Just over a hundred years ago,

we invented a wonderful new material

that could be moulded into all kinds of shapes.

And we took great trouble to ensure

that it was hard-wearing, waterproof,

and virtually indestructible.

Now, every year,

we dump around eight million tonnes of it into the sea.

Here, it entangles and drowns vast numbers of marine creatures.

But it has even more widespread and far-reaching consequences.

A pod of short-finned pilot whales.

They live together in, what are perhaps,

the most closely-knit of families in the whole ocean.

Today, in the Atlantic waters off Europe, as elsewhere,

they have to share the ocean with plastic.

A mother is holding her new-born young.

It's dead.

She is reluctant to let it go

and has been carrying it around for many days.

As plastic breaks down, it combines with other pollutants

that are consumed by vast numbers of marine creatures.

In top predators like these,

the toxic chemicals can build up to lethal levels.

It's possible her calf may have been poisoned

by her own contaminated milk.

Pilot whales have big brains.

They can certainly experience emotions.

Judging from the behaviour of the adults,

the loss of the infant has affected the entire family.

Unless the flow of plastics into the world's oceans is reduced,

marine life will be poisoned by them for many centuries to come.

The creatures that live in the big blue

are, perhaps, more remote than any animals on the planet.

But not remote enough, it seems,

to escape the effects of what we are doing to their world.

Next time, we journey into the bountiful green sea.

These are enchanted worlds,

home to strange creatures...

...where only the most ingenious will triumph.