we finally ventured to the moon.

For the very first time, we looked back at our own planet.

Since then, the human population has more than doubled.

This series will celebrate the natural wonders that remain,

and reveal what we must preserve

to ensure people and nature thrive.

The polar regions of our planet may seem beyond the reach of most of us,

but they are not beyond our influence.

We, unintentionally, are changing these frozen worlds,

and these changes will not just affect the poles

but the whole planet.

Antarctica.

The southern end of our planet, and the coldest place on Earth.

It has been frozen for 30 million years.

In its center, the covering of ice is over four kilometers thick.

Entire mountain ranges are buried beneath it.

Here, it is so cold that each winter

19 million square kilometers of ocean freezes,

more than doubling the size of the ice cap.

But when, in spring, the sea ice melts,

life returns once again to the continent's shores.

The peninsula that stretches north towards South America

is the first part of it to be released from winter's grip.

As the sea ice breaks up,

life returns.

[gulls cawing]

There is a greater variety of living creatures here

than anywhere else in Antarctica.

Gentoo penguins spend most of their lives at sea,

but now, in spring, they have to come ashore to breed.

They're the fastest of all penguins in water,

but on land, life takes on a slower pace.

It's an uphill struggle to reach their nesting grounds...

[penguins honking]

...but penguins never give up.

The paths they follow, carved by thousands of tiny footsteps,

lead to Antarctica's rarest commodity.

Bare rock.

Less than one percent of Antarctica is ice-free,

and these rocky patches

are the only places where gentoos can lay their eggs.

It's been a 30-minute struggle,

but he's almost at the top, where his mate is waiting for him.

[honking]

He presents her with a stone,

a gift to improve the nest and so win her favor.

[both honking]

He's back just in time.

There's another mouth to feed.

[chirrups]

Gentoo penguins used to be rare in this part of Antarctica,

but now, as temperatures rise, their numbers here are increasing.

Sea ice may appear to be flat and lifeless,

but beneath it there is another world.

As the ice melts, the light filters through,

and algae, that have been trapped within the ice all winter, flourish.

This upside-down world

is the polar equivalent of the great grasslands.

The ice is the soil upon which plant life grows,

and the herds of grazers soon arrive.

Antarctic krill.

Trillions of them swarm in the waters around Antarctica.

Protected by the ice above, they graze on algae whilst the sun shines.

But the good times don't last long.

Once all the ice has melted, the krill lose their protection,

and predators appear.

Penguins may have lost their ability to fly,

but they still form flocks, as so many birds do.

The longer they dive,

the longer it takes them to catch their breath.

Ruffling their feathers traps air, which makes their bodies more streamlined.

One last breath...

and the whole group dives down together.

The gentoos follow the krill down to depths of over 200 meters.

What a flock can catch in one dive,

a giant traps in a single mouthful.

Humpback whales from the tropics

have traveled over 8,000 kilometers to get here.

This one chooses to feed alone.

This technique, part lunge, part pounce,

only works when the krill are close to the surface.

As more humpbacks arrive, they begin to work in teams.

By lunging together,

the overspill from one huge mouth is not lost but collected by another.

Some humpbacks regularly work in an even more coordinated way.

They do so when krill has descended to some depth.

Once in position, they blast air from their blowholes.

Working together,

they create a curtain of bubbles around their prey.

As the spiraling net tightens, the krill are driven closer together,

and then the whales lunge upwards and collect them.

Almost all humpback whales in the southern hemisphere

come to the Antarctic to feed on krill.

Since the ban on commercial whaling, their numbers have recovered dramatically,

but their food supply is now under threat.

In the last 50 years,

with warming temperatures and disappearing sea ice,

krill stocks in this part of the Southern Ocean

have more than halved.

Other sea hunters come here seeking bigger prey than krill.

Antarctica's top predators.

Orca, killer whales.

Over half of the world's orca patrol these polar seas.

This particular pod specializes in hunting penguins.

They shift into stealth mode.

Hunting without making a sound, so that their prey won't hear them coming.

A penguin can outmaneuver a single killer whale,

but it can seldom escape a pod.

This hunt may seem cruel,

but important life skills are being passed down the generations.

From killer whales to krill,

all life here ultimately depends upon Antarctica's sea ice.

The seas that surround Antarctica are the roughest on our planet.

These churning waters draw up nutrients from the deep

and create vast, rich fishing grounds for nomads like the wandering albatross.

Beneath the waves, currents are flowing away from Antarctica,

transporting nutrients across the planet, fertilizing the oceans

and helping to regulate global temperatures.

The island of South Georgia.

It's far enough north

to be beyond the reach of the sea ice even in midwinter,

and many animals live here the year round.

This wandering albatross chick has been sitting on its nest all winter,

and is entirely dependent on its parents returning with food.

It may have to wait a few days between meals, or even a couple of weeks,

but at least it's not the only one who's waiting.

Albatross chicks need to spend a whole year growing

before they're ready to take their first flight.

And then they spend several years at sea before they return to land.

[honking]

Coming in to land  on three-meter wings takes some practice.

Albatross don't have reverse thrusters.

[chirping]

The chick  is this female's first.

She's been raising it for six months, and has another six to go.

Her chick is smaller than it should be,

a sign that her mate has not been returning with food.

He may well have been one of the many that die,

entangled on longline fishing lines.

South Georgia is an ideal place for wanderers to raise their young,

but now fewer adults are returning here to nest,

and in recent years,

the breeding population has declined by 40 percent.

For now, waiting on its nest, the chick is safe.

There are no land predators here.

But the same cannot be said for South Georgia's seas.

Giant kelp flourishes in these ice-free waters.

The perfect place for an ambush.

Leopard seals overwinter here to escape Antarctica's frozen ocean.

They should have returned south by now,

but this year they've stayed longer.

And they've delayed their departure for good reason.

There are king penguins here.

Each is a substantial meal,

if you can catch it.

Returning to feed their chicks,

the kings have no choice but to run the gauntlet.

Kings rarely porpoise like this.

It uses more energy, but it's faster than traveling underwater.

It's hard to select a moving target in a panicking crowd,

but if there are stragglers, the odds change.

Razor-sharp teeth versus a stabbing beak.

It's stalemate.

Both are exhausted after the chase.

The hunter decides that the meal is not worth the fight.

[cawing]

For the penguin, it was too close for comfort,

but that was just the first hurdle in getting back to feed its chick.

[cawing]

It isn't going to be  a walk in the park.

[elephant seals snoring]

Luckily, penguin is not on their menu.

Still, it's best to let sleeping giants lie.

[penguin honks]

Elephant seal females  don't take kindly

to being disturbed from their slumber,

but it's the four-tonne males you need to look out for.

[rasping grunting]

They are fighting for access to the females.

Getting bulldozed under angry blubber is to be avoided at all costs.

An escape route.

Time to run for it.

At last, he's reached the nursery.

Now comes his final challenge.

Finding his chick in the crowd of half a million.

He calls...

[cawing]

...and listens  for a recognizable reply.

[chirping]

His chick could be another mile or two away,

at the back of the colony.

But that's the trouble with chicks.

They're never where you left them.

Astonishingly, in all this racket,

a parent and its chick can recognize each other's calls.

[chirping]

The things parents go through to deliver a meal.

If only they knew.

For such a small island,

South Georgia supports an extraordinary quantity of life.

All these creatures ultimately depend on the krill

that develops under the sea ice around the Antarctic peninsula.

With sea ice disappearing,

so South Georgia's future is far from certain.

At the other end of the planet, in the north,

the effects of climate change are being felt even more intensely.

The Arctic is not a continent surrounded by sea

but a frozen ocean ringed by land.

Here, in spring, the sea ice is still at its maximum extent.

It's a vast, firm hunting ground for the planet's largest land carnivore.

A male polar bear in search of his next meal

on the frozen fjords of Svalbard in the high Arctic.

He's looking for seals, which make up the bulk of his diet.

He must eat two-thirds of the food he needs for the whole year

before the ice melts and his hunting platform disappears.

The ringed seal mother had no choice but to leave her pup alone on the surface.

In the past, she would have built a den inside a ridge on the ice,

but now the sea freezes so late that the ice is flat,

and she had to leave her pup out in the open.

These changes in the ice may benefit the bears for now,

but in the long term,

a crash in their most important prey will follow.

This new, flatter ice makes life difficult for the bears, too.

There's nowhere for them to hide in order to creep up on a seal.

And for a mother with a young cub in tow, being stealthy isn't easy.

The cub doesn't yet appreciate the need for care

when sneaking up on a seal.

With fewer seals hiding in dens,

learning this pounding technique, once so useful,

is now almost pointless.

Still, at least he's discovered what a seal hole looks like..

His mother knows that overcast conditions are good for hunting,

and he knows when she's serious.

Adult seals make a much better meal.

Her cub carefully treads in her footsteps,

and takes care not to break her outline.

Neither must put a foot wrong.

They must move as one.

Most seal hunts end in failure.

The bears must keep searching.