Original subtitles

Just beyond the horizon...

...lies a vast wilderness.

The open ocean.

It covers over half the Earth's surface...

...and is what makes our planet blue.

It may look empty...

...but hidden here are the ocean's

most spectacular animals.

WHALE BLOWS

A blue whale.

WHALE BLOWS

At 25 metres long,

it's the largest creature that has ever existed.

Somehow...

WHALE BLOWS ...it must collect

around ten tonnes of food every day

from this immense marine desert.

There are great riches here.

But only for those special few

who know where to find them.

WHALE BLOWS

Where coastal waters end,

the open ocean begins.

This group of sea lions

left their home on the coast of Central America

several days ago.

They have travelled far into the Pacific

in their search for food...

...following routes established over generations.

SEA LIONS CALL

They scour over 1,000 square miles of water.

With whiskers over 20cm long

and highly sensitive hearing,

they can detect the faintest disturbance in the water.

Sardines and scad...

...attracted here by nutrient-rich waters.

But they aren't easy to catch.

A school's defence relies on near-perfect synchrony -

thousands of fish moving as one.

The sea lions struggle to pick them off.

But out here,

others can.

Mahi-mahi.

Open ocean predators.

As they approach, they change colour from silver to green.

It's a sign that the hunt is on.

They charge in at high speed.

For them, it's easy.

The sea lions watch on...

...outpaced and outmanoeuvred.

Now, hundreds of black skipjack tuna arrive...

...pinning the school to the surface.

But that makes it vulnerable to an aerial attack...

...from pelicans and frigate birds.

Supper is vanishing fast.

Under intense pressure,

the school's defence is failing.

It splits into two.

Tuna chase the larger group...

...whilst the sea lions and mahi-mahi pursue the rest.

The mahi-mahi gorge themselves.

All seems lost for the sea lions.

But this is the very moment they've been waiting for.

The fish are exhausted...

...and can be picked off with ease.

Sardines and scad store fat for long migrations...

...so they are much more nutritious

than fish found near the shore.

Once satisfied,

the sea lions can begin the journey

back to their coastal home.

Some animals here aren't just visitors.

They spend their entire lives in the open ocean.

In the Atlantic,

500 miles from the coast of Morocco...

...rough-toothed dolphins.

CLICKING AND WHISTLING

Their kind have roamed these waters

since prehistoric times...

...but we know almost nothing about them.

WHISTLING

They're constantly on the hunt for their next meal.

Finding it takes perfect teamwork

and patience.

They come together...

WHISTLING

...form a straight line...

WHISTLING

...then all fall silent.

The leader sends out a stream of clicks.

CLICKING

CLICKING

CLICKING

Everyone listens...

CLICKING

...waiting for an echo to return from the blue.

CLICKING MUFFLED ECHO REVERBERATES

Something is out there.

Time to investigate.

CLICKING AND WHISTLING

CLICKING AND WHISTLING

A loggerhead turtle is no meal for a dolphin.

But it does have some travelling companions.

Pilot fish.

They use the turtle as a shield to protect them from predators.

The dolphin just needs to work out

how to separate them.

CLICKING AND WHISTLING

Blasts of bubbles

flush the fish away from their bodyguard.

CLICKING

Got one!

This ingenious hunting strategy

is unique to these dolphins.

By working together and taking it in turns,

they can feed the whole pod.

WHISTLING

Rough-toothed dolphins travel thousands of miles

every year in search of prey.

But some animals can't actively swim.

They must take advantage of the ocean's natural forces.

These creatures are called by-the-wind sailors.

Just 7cm long,

they have a stiff sail that catches the breeze...

...and each can journey up to six miles a day

on wind power alone.

Below the surface...

...tiny stinging tentacles catch plankton

as it drifts by.

Their entire life is one of chance.

Rising from the depths beneath them...

...a Mola mola

the size of a bathtub.

Normally, they hunt some 200 metres below the surface.

But these delicacies are too good to miss.

It can devour hundreds.

These waters provide more than a feast.

Floating kelp is a rare oasis...

...attracting Mola mola from far and wide.

Here, halfmoon fish offer a special service.

They can accumulate

more than 40 different kinds of parasites.

For the halfmoons,

each one is a nutritious meal.

And for the Mola mola,

it's a place to be pampered.

After being cleaned, it's time to sunbathe,

earning this species the nickname of sunfish.

Heat warms their muscles

so they're ready to hunt in the colder depths again.

The big blue may seem a featureless place.

THUNDER ROLLS

But out here, there are invisible forces.

Ocean currents.

They flow like rivers.

Some swirl around,

forming vast, spiralling eddies.

Others stretch thousands of miles.

Together, they transport

over a billion litres of water every second.

And in the Indian Ocean...

...hitchhikers, too.

A Smith's swimming crab.

No bigger than a bottle-top.

Whilst most crabs live close to the coast,

this drifter has spent its entire life

riding currents...

...and feeding on plankton.

And it's not alone.

With predators few and far between,

the open ocean is an excellent home.

But these crabs have no control over their destiny...

...and obstacles can appear without warning.

A coral atoll in the Seychelles.

For some...

...it's the last thing they will ever see.

There are hundreds of predators here.

This crab may have never felt solid ground.

Instinct tells it to take shelter.

But even hiding isn't safe.

It needs to get back to the open ocean.

Tiny claws are a poor defence.

If it can just make it to the surface,

it will catch the current.

Within a few hours,

it will be swept miles from danger...

...back out to sea.

In the Atlantic,

the flow of water is so strong,

it creates something unique.

The Sargasso Sea is the only sea on Earth

bordered not by land, but by ocean currents.

Over a million square miles of water,

held in place for millions of years.

Within it, gigantic mats of floating seaweed

called sargassum.

But in the year 2010,

something unusual happened.

It's thought that the currents shifted...

...allowing sargassum to escape.

Data from space

shows that this runaway seaweed

now stretches 5,000 miles...

...all the way from Africa to Central America.

It's reaching the coast of Mexico,

where it's having a dramatic impact

on the animals that depend on its beaches.

Here, under the cover of night,

a tiny green turtle hatchling

is emerging after 60 days in an underground nest.

Taking a first glimpse of the outside world,

the hatchling heads straight towards the sea,

as its kind has done here for millions of years.

50 siblings emerge just behind.

But today, the ocean delivers dangers

their ancestors would never have encountered.

Over a million tonnes

of plastic washes up on beaches every year...

...creating deadly traps.

But now there's a newer challenge.

Sargassum.

In 2025,

over 45 tonnes of it

was washed up on this beach alone.

Climbing the tangled mass is exhausting.

A journey that should take two minutes...

THUNDER CRASHES

...can now take over ten times as long.

One mountain is followed by another.

WAVES LAP Close to the water...

...the weed moves with the waves.

This creates deep, rolling crevasses.

The crushing sargassum...

...is a killer.

GURGLING

CRACKLING

These turtle hatchlings stand little chance.

Not without help.

Nearly all are rescued by conservationists...

...and taken to the safety of the sea.

The open ocean

is where they will spend the next 80 years.

Out here, some wanderers are found above the waves.

Frigate birds spend weeks at sea,

even sleeping on the wing.

But sometimes even they must find solid ground.

Isla Isabel - a remote island in the Pacific.

Over 10,000 gather here for the breeding season.

This female has an eight-week-old chick.

The father has abandoned them,

so she's left alone to raise her youngster

for over nine months.

Every week, the chick needs to consume

almost its whole body weight in fish...

...so it is always hungry.

BIRD CHIRPS

But finding food isn't easy.

All frigate birds lack waterproof feathers.

If they tried to dive for fish, they would drown.

Others on the island seem to find parenthood easier.

Blue-footed booby couples work as a team...

...and seem to have no difficulty

in finding food for their offspring.

For them, fishing is not a problem.

They can dive deep...

...for gullet-fuls of fish.

But their success is this mother's opportunity.

Known as a pirate bird...

...frigates are thieves.

A tug on the tail feathers

should make the booby throw up its fish.

BIRDS SCREECH

But gulls are following.

Unafraid of the water, they can get closer.

The booby ejects its fish...

...and the frigate's prize is stolen.

GULLS SQUAWK

Back at the nest, the chick is growing hungrier.

But this wily mother has another plan.

Out at sea,

fishermen are also attracted by the rich waters.

Pirates don't always need to steal.

These fishermen...

...are happy to share.

For more than a century, enterprising mothers

have given their chicks a better chance of survival

by following fishing boats.

Today, we aren't just fishing the ocean.

We are farming here, too.

In the Atlantic, hundreds of miles from the mainland,

14 nets...

...some holding 300,000 sea bream.

They will grow for a year before being harvested.

With many wild fish stocks overexploited,

offshore farms could be one of the few long-term ways

to provide food for our growing population.

But out here, opportunities like this don't go unnoticed.

It's this loggerhead turtle's lucky day.

A seemingly endless supply of food.

There's just one problem.

The turtle is on the wrong side of the net.

A dead fish should be an easy target...

...were it not for the stubborn wrapper.

Another loggerhead turtle.

No-one likes being interrupted at dinner.

A bite on the bottom should do the trick.

With the intruder seen off...

...he can get back to his meal.

In a place where food is often hard to find...

...an offshore fish farm has plenty...

...so this turtle may spend many years here.

From fishing to farming,

our activities have the power to shape life in the ocean.

When we choose to protect them, animals thrive.

WHALE BLOWS

Blue whales,

once hunted close to extinction in the 20th century,

are now protected.

Slowly, their numbers are recovering.

Today, up to 25,000 of these giants

roam our ocean once again.

And we are now witnessing behaviour

not seen for decades.

Each winter, hundreds come from the open ocean

to feed in the waters off Mexico.

But they're being followed.

Killer whales.

A close-knit team, over ten strong,

of whale-hunting specialists...

...with their sights set on the biggest target of all.

A fully grown adult blue whale.

The killers are driving him towards the shore

in the hope of catching him.

He appears unaware of the danger ahead.

It seems the battle is over.

But against all odds,

this 80-tonne adult whale

has managed to turn around.

After four hours of pursuit...

...the balance has finally shifted in his favour.

He is a master of the high seas,

and has far greater stamina than the killers.

If he keeps swimming until night falls...

...they will be forced to abandon their hunt.

This vast blue wilderness is still giving up its secrets.

If we can protect it, and the animals that are hidden here,

who knows what wonders we may yet discover

in the open ocean?

Finding something 5,000 miles wide

should be easy.

But this is the open ocean,

where anything can hide.

We're starting to see little patches of the sargassum,

which is a good sign. It starts to build up, little by little.

The team is searching for a colossal amount

of sargassum seaweed

that is being swept across the Atlantic.

Taking off in three, two, one, go.

Drones make it easier to find.

Go in left. Right there is a big one.

Their aim is to film the hidden world

beneath the weed.

Out here...

...these drifting rafts serve

as an important sanctuary for hundreds of species.

It's quite special to see these ecosystems where

they are so important.

They provide habitat for so much life.

But once they hit shore, they can be quite devastating.

Cinematographer Mark Payne-Gill

wants to film the impact of the sargassum

on turtle hatchlings.

We've got to use these specialist low-light cameras.

Right now I'm filming in normal mode.

If I flip this switch...

...and I'm now seeing into infrared.

The difference is quite incredible.

It's like almost full-on daylight,

apart from the fact I can see stars.

The difficulty is, even with this camera

that can see in the dark so clearly,

we can't. There's lots of nests

potentially hatching all the time,

but it's hard to predict where that's going to be.

Dedicated conservationist

Claudia Leon Garcia and her team

keep track of 200 turtle nests here,

and they know the signs to look for.

Sometimes it's difficult to know

when they are going to hatch.

Sometimes we can see, like, a drop in the sand,

other times we can see the tiny heads close to the surface.

Small movements in the sand is a sign

that we know that we are going to have hatchlings.

Mark sets up at a nest that's due to emerge.

So, we've got this nest here that's all poised to erupt.

There's two little heads poking out.

These could hatch any time,

but when it happens, it'll be a mad rush.

Hatchlings can take up to two days to leave the nest.

MARK YAWNS

THUNDER CRASHES

This is slow TV.

He could be in for a long night.

To increase Mark's chance of success,

the team tracks the progress of other nests.

We're just sitting by a nest,

waiting for it to hatch.

It looks like they've fallen asleep.

Come on, turtles!

They're so sleepy.

After four hours, Mark is still waiting.

But there is movement by Harry.

This is nest 117.

I'm not sure how many eggs were down there,

but there could be at least 100.

And so far we've only seen one.

There he is again!

A raccoon right there. Ah-ah-ah!

Get back!

They're like little fluffy bears,

but they're trying to eat all the hatchlings.

They're just waiting.

Come on, guys. We need to go.

I feel like I'm being watched.

RACCOON SNUFFLES

They're literally behind me.

There's three of them.

RACCOON SNARLS

Don't know what to do.

Psst! RACCOON CHITTERS

Get back!

A conservationist soon turns up

to protect the turtles.

A highly sophisticated raccoon barrier

is all that's needed.

RACCOON CHIRRUPS

Plastic crates reduce predation by more than 60%.

For Mark, patience finally pays off.

Another one.

This is it. Yeah, there's more coming up now.

Amazing.

Wow. Loads of them!

It's brilliant.

They're just all piling up now.

And they look really dopey, but suddenly

they just start racing off.

OK, I've got to see if I can track with them.

All right.

This is not good.

They're really struggling, as you can see.

Look at this poor fellow here.

That's horrendous.

This sargassum is not going to go away.

It's something that we need to face it

and to make some solutions.

Sargassum doesn't just affect wildlife.

The toxic gases it releases when rotting

can impact human health, too.

But whilst making this series, the Mexican government realised

that sargassum is a valuable resource.

Each year, along with many Caribbean nations,

they plan to harvest it for use in cosmetics,

construction materials and bioplastics...

...helping the people who live there,

and making the journey easier for turtles.

Back on the filming beach,

Claudia and her team

carefully release any rescued hatchlings

in clearer areas.

This is the best part of the job for me.

Release these babies in a safe spot

after all the challenges that they have.

In 2025, the conservation team here

helped more than 10,000 turtle hatchlings reach the sea.

You've got to admire these little creatures

because they're so determined to get to the water.

Off he goes.

Brilliant to see him actually make it.

Hurrah!

Next time...

...the seasonal seas.

Those who can survive relentless change...

...are rewarded with the most bountiful waters on Earth.

Explore our changing oceans with The Open University's free poster.

To order, call...

...scan the QR code or visit...

♪ Into the deep, I want to see beyond the light

♪ Look to the blue, it's calling you

♪ Oh, what a sight

♪ I hear the seas cry. ♪