Original subtitles

In our most remote ocean...

...over a thousand miles from the nearest continent.

Zavodovski Island.

It's prone to volcanic eruption,

sparse in vegetation,

and lashed by storms.

Yet, Zavodovski is home to over 2,000,000 chinstrap penguins.

They're here because the surrounding waters provide

some of the best fishing on the planet.

These youngsters are used to having their meals delivered.

The challenge is for those making the delivery.

A mother chinstrap,

full of krill for her chicks.

Just one thing stands in her way.

The sheer cliffs that surround this island fortress.

It's hard enough for those trying to get down.

How will she get up?

She could scramble up the rocks.

Maybe the solution is not to fight the ocean,

but to ride it.

Go with the flow.

But the flow runs both ways.

The whitewater shows no mercy.

To have any chance of getting food to her chicks,

she'll have to tighten her chinstrap...

...and brace herself

to ride the biggest wave of all.

Made it!

Now the final challenge...

a one-mile climb up the side of an active volcano

to deliver her precious cargo.

The mighty chinstrap proves that anyone tough enough to survive gets rewarded,

in this, the most isolated ocean of all.

The Southern Ocean completely surrounds Antarctica.

It's home to the coldest waters on our planet,

where winters can drop below -50 degrees,

and winds reach almost 200 miles per hour.

This remote ocean is where the global ocean current is most powerful,

long protecting hidden worlds.

At the outer fringes of the Southern Ocean lies the coast of southern Australia.

Its cold waters are home to extraordinary species.

Many found nowhere else.

Just offshore lives the handfish.

One of few fish that prefers to walk rather than swim.

And this spotted handfish is one of the rarest fish in the world.

Fewer than 3,000 exist...

which makes what she's guarding extremely precious.

She's laid her eggs in the only place she can.

The stem of a sea tulip.

Raised off the silty bottom, they're anchored against the current.

But this does dangle them in clear view of hungry neighbors.

Luckily for her young,

she's one tough mother.

Tail whips...

...take care of hermit crabs.

Giant sea slugs are bitten into submission.

She has an answer for every invader.

But even she has a nemesis.

A monster sea star, ten times her body weight.

Each of its hundreds of feet can feel,

taste, and smell.

And they've locked onto the scent of her eggs.

Biting and pushing are futile.

It's unstoppable.

But she can't give up.

So, she makes a risky move.

Offering herself to the sea star...

...in a desperate attempt to lure it away from her eggs.

It seems like a remarkable sacrifice.

But then...

...she gives it the slip.

Thanks to resilient moms like her,

spotted handfish still have a fighting chance.

In the Southern Ocean, you have to do whatever it takes to survive.

And the further south you go, the more hostile the ocean becomes.

The great current flows full circle around the Southern Ocean.

A rotating wall of water,

up to two-and-a-half miles deep and a thousand miles wide.

It's known as the Circumpolar Current.

And in its path

is South Georgia,

one of the most remote islands in the world.

Yet, for a few weeks every spring,

it becomes the busiest place of all.

A density of coastal wildlife found nowhere else.

It's the breeding season,

and every inch of this beach is contested by giants.

Bull elephant seals,

onshore to battle for breeding rights.

They can grow up to 20 feet long and weigh more than 8,000 pounds.

These enforcers are as big and bad as they come.

But even the ultimate tough guy was small once.

A baby born just hours ago.

He'll face many challenges in his quest to become a giant himself.

First, he needs to fill out his oversized birthday suit

with his mother's rich milk.

Because he has competition.

A south polar skua...

...who has no qualms about stealing from a baby.

This skua's off to find an easier target,

and the pup's learned a lesson.

Stay close to Mom.

But his next test won't be so easy.

With the beach overcrowded, tensions are growing.

Giant males stop at nothing to defend their territories.

They don't care who gets caught in the crossfire.

Being crushed is the number one cause of death for newborns.

But today, this little pup has been given a second chance.

And so far, Mom's never left his side.

Over the new three weeks, he'll put on 150 pounds,

and then everything changes.

Because one day, he wakes up to a terrible shock.

Mom is gone.

Having given him nearly half her body weight in milk,

she has to return to the ocean to feed.

He can't follow because he can't swim.

He needs to find a safe place to learn.

But, he's easily... distracted.

The seminal moment for every elephant seal pup is

their first contact with water.

On entry, an extraordinary reaction occurs.

He holds his breath.

His heart rate slows.

Blood diverts to his brain.

It's the first step on his journey

to becoming a marine mammal.

But he still has a lot to learn.

After a month of practice in the shallows with other pups,

he's ready to enter the ocean for the first time.

Over the next few years,

he'll learn to hold his breath for more than two hours

and dive to a depth of over 6,000 feet.

And one day, he'll return to these shores

as the largest, toughest seal in our oceans.

From South Georgia,

the great current surges on at five billion cubic feet per second.

Driven by ferocious winds and the turning of the earth,

it becomes the most powerful movement of water on the planet.

A gigantic barrier to the other oceans.

And at its center is the most isolated continent of all.

Antarctica.

Over five million square miles of frozen wilderness.

Many of the continent's vast glaciers extend out from land

as floating ice shelves on the ocean's surface.

At the ocean's southernmost point,

the current pushes under the Ross Ice Shelf,

which is larger than the state of California.

The ice here can be over 800 feet thick.

And yet, where the current flows,

there is life.

Invertebrates have existed down here, seemingly unchanged,

for millions of years.

All of them clamoring for their next meal.

Including an Antarctic sea spider.

He's the size of a dinner plate,

and may not have eaten for several months.

With all the competition here,

that's unlikely to change anytime soon.

He needs to find somewhere where he has the advantage.

A place where the water is so cold, it's on the brink of freezing.

Where ice crystals can grow on any surface they touch...

...forming ice caverns.

But their beauty hides their danger.

Because any creature that touches the ice here

can become trapped.

But all that danger presents a risky opportunity

for the sea spider.

Using his long, spindly legs,

he tiptoes across the ice...

being careful not to trigger its grasp...

and slurps up his jelly dinner.

Who needs a web when you've got an ice cavern?

These super-cold conditions are sustained by the Circumpolar Current,

which acts as an icebox, holding in the cold.

And as winter descends,

the ocean's surface around Antarctica freezes into a vast sheet of solid ice.

Covering up to seven million square miles,

it's the perfect cover for the most important species

in the Southern Ocean.

One with immeasurable power over others.

Antarctic krill.

Trillions of these tiny, shrimp-like crustaceans feed on the algae

that grows beneath the sea ice.

Their combined weight is thought to be over 400 million tons.

The greatest mass of any wild animal.

When summer arrives, the ice melts,

and krill hunters swoop in.

From resident gentoo penguins

to great whales who've traveled thousands of miles.

It's the great annual feast.

Then, winter comes and the whales depart,

and the sea freezes over again.

Only the hardiest remain behind.

Who said they can't fly?

Emperor penguins.

After two long weeks at sea,

these parents need to return to their chicks,

who are waiting on thicker sea ice several miles away.

So, despite just completing an epic swim,

they quickly transition to the track.

All fully equipped with the technical gear required to survive in extreme conditions.

Thick blubber protects them from the cold.

Waterproof plumage keeps them dry.

Even their legs are feathered.

Every adaptation refined by evolution to endure our harshest ocean.

They've almost made it home safely...

...when the unexpected

...puts a stop to everything.

The life of every emperor penguin rests on the presence of this sea ice.

So, while a small crack may seem harmless to us,

to them it signals a frightening change.

And while they may not know why, they're right to be concerned.

The climate crisis has reached our oceans' most isolated region.

The penguins' world is suffering from record-breaking heatwaves,

up to 50 degrees Fahrenheit above normal.

If the sea ice melts before their chicks have developed waterproof coats,

they'll drown.

Multiple colonies have already suffered premature ice loss,

causing the deaths of tens of thousands of chicks.

For nature's toughest endurance specialists,

this may be the most difficult challenge of all.

Scientists fear that if the warming continues,

by the end of the century we may have a Southern Ocean without emperor penguins.

As the current continues its journey,

its path is narrowed by the Antarctic peninsula,

where fierce winds and strong currents

force deep water to well up to the surface.

This water, which originated in the Pacific and Atlantic Oceans,

is warmer today than ever before.

The upwelling acts as a secret entrance,

allowing warmth to infiltrate the Circumpolar Current,

further adding to the melt.

With its defenses breached,

summers have become so extreme

that sea ice around the peninsula

has become increasingly scarce.

The greatest ice loss since records began.

Without ice to feed beneath,

most of the krill have abandoned these waters,

along with many of the animals that depend on them.

Some of the last remaining sea ice is protected deep within the fjords,

and clinging on is an ice-dependent creature

who's not going down without a fight.

The crab-eater seal.

This rugged young male is as tough as they come.

He's a fighter.

And he's got the scars to prove it.

Born on the sea ice, he's never set a flipper on land.

But lately,

his world has become unrecognizable.

He's found one of the few remaining pieces of ice he can haul out on.

And he's joined by another species of seal,

a Weddell,

who's also seeking refuge.

The bay is unusually quiet.

Orca.

Antarctica's killer whales.

But these are not any old killer whales.

They are seal hunters...

and they've covered all the exits.

For the seals, the sea ice has become a life raft.

Luckily, the sharp edges of the ice

prevent the killer whales, and their thin skin,

from getting too close.

As long as the seals stay on their raft, they're safe.

But this pod isn't going anywhere.

Working together, they channel their power to create a single, large bow wave.

The crabeater gets right back on,

with the Weddell trailing behind.

But the wave hunters

are just getting started.

The Weddell, slower-moving and with more body fat,

is the easier target.

And unlike the crabeater,

Weddells aren't natural-born fighters.

It's a quick death for the Weddell.

But the hunt

is not over.

These killer whales are struggling to find enough food.

The fiery crabeater...

...is not their first choice of prey.

But without much sea ice,

they'll take what they can get.

As they regroup,

he seizes the moment to slip away.

A piece of ice, jammed between two others, protects him from their attack.

But in an astonishing display of tactical intelligence,

the killer whales begin to free the ice.

This larger piece should put an end to it.

But the hunters aren't finished.

Using a series of subsurface waves,

they dismantle what remains.

Now they work from below

to tip the ice over.

But the crabeater still has some fight left.

But no matter how hard he tries,

he can't shake them off.

Until there's nowhere left to run.

After four hours of continuous assault,

he is totally exhausted.

But the waves are getting smaller.

And less powerful.

The killer whales are exhausted too.

You win.

The killer whales head out into an uncertain future.

There are only around 100 of these specialist wave hunters in the world.

As the sea ice declines, so do their prospects.

If they lose their ability to hunt like this,

we'll lose them too.

All Antarctic wildlife depends on the sea freezing.

Even deep beneath the surface.

Because each year, as it chills,

250 trillion tons of surface water plunges into the depths,

taking the great current with it.

At 3,000 feet, darkness and cold combine with enormous pressure

to slow the pace of life.

Here, at the bottom of the Southern Ocean,

are the oldest living animals on Earth.

This volcano sponge was alive before the pyramids were built.

They can live for 15,000 years.

The Southern Ocean is the birthplace of countless deep-sea creatures.

And by riding the great current, they've spread out from Antarctica.

So that today,

the weird and the wonderful

have made it into the depths of every ocean.

Revealing that, thanks to the current,

the Southern Ocean is more connected than we've ever imagined.

It's the planet's beating heart,

pumping out a cold, deep flow of water which powers the global ocean current...

...on its 75,000-mile journey from one ocean to the next.

Forming one interconnected body of water

that is our one great ocean.

But because the current connects every ocean,

what we do anywhere

can impact lives everywhere.

A piece of trash carried by the current can end up on the other side of the world.

Thanks to our actions, our oceans are now the hottest on record.

As the current spreads this heat around the world,

fish populations move,

making them harder to protect.

And ice melt is accelerating...

...which could threaten the future of the great current itself.

Without it,

our weather would become even more extreme.

If we don't keep the oceans healthy,

they may no longer sustain us.

Yet here, in the Southern Ocean,

we've shown that we can step up when it matters most.

This southern right whale is living proof.

We once hunted her kind until there were fewer than 100 breeding females left.

But at the eleventh hour, countries around the world pulled together

to protect the great whales.

And Southern rights have made a remarkable recovery.

Today, thousands of them journey from Antarctica

to breed along the southern coast of Australia.

Proving that if we give ocean life a chance, it can bounce back.

And if we can help save a species from the brink of extinction,

surely we can work together

to bring our oceans back to health.

More and more of us are taking action.

Cleaning up our waste.

Pledging to reduce emissions.

Protecting species...

and their homes.

If we act now,

we still have the power to help heal our one great ocean.