Both poles of our planet are covered with ice.
They're the largest\nand most demanding wildernesses of all.
Nowhere else on Earth\nis seasonal change so extreme.
It causes the ice\nto advance and retreat every year.
And all life here is governed by that.
When the first polar explorers headed south,
giant cathedrals of ice marked\ntheir entry into uncharted territory.
Passing the towering spires,
they must have wondered\nwhat unearthly sights lay in store.
As they battled on,\nthe ice became increasingly dominant,
but nothing could have prepared them\nfor the ice world that finally loomed into view.
Terra incognita, ''the unknown land''.
At the southernmost extreme of our planet,
the continent of Antarctica is as large\nas the United States of America.
Ninety per cent of all the world's ice is found here.
This frozen world is largely deserted\nuntil the start of spring.
Adelie penguins in a hurry.
The clock is ticking.
Instead of waiting for the summer melt,\nthe new arrivals hasten south over the frozen sea.
They have come here to breed,\nbut polar summers are so short,
they must be in position before the thaw starts.
As the sea ice retreats,\nlife can journey farther south.
Antarctic waters are so rich that visitors come\nfrom far and wide to harvest them.
Vast numbers of chinstrap penguins\ncome ashore to breed.
No bird will lay their eggs directly onto ice
so bare rock is a vital commodity.
The best patches are worth the climb.
The cliff tops are soon stained pink
with the droppings of tens of thousands\nof nesting penguins.
0nly in a land almost entirely covered in ice\ncould bare rock be reckoned an oasis.
Some will travel\ninto the heart of the continent to find it.
These are nunataks,\nthe exposed peaks of vast mountain ranges
buried in ice over a mile deep.
The eerie silence here is only broken in spring.
The snow petrels have arrived and are courting.
Antarctic petrels now join\nthe most southerly bird colony on Earth.
The birds have flown inland\nfor over 300 miles to reach this breeding site.
0nce their eggs have hatched, they will be forced\nrepeatedly to make the 600-mile round trip
to gather food in the ocean.
First, though, valuable nesting places\nmust be defended from property thieves.
After laying their eggs,\nthe petrels take time out to clean their plumage.
The south polar skua is a formidable opportunist.
But the skuas have not chanced upon the petrels,
they've been waiting for them.
These birds do not need to go\nto the ocean for their food.
The skuas can survive further south\nthan any other predator
by exploiting the petrels' desperate need\nfor bare rock.
Even at the height of summer, less than\nthree per cent of Antarctica is free of ice,
and nearly all of that exposed rock\nis found in one place,
the Antarctic Peninsula.
Its long arm extends further north\nthan the rest of the continent,
so spring arrives here first.
The thaw unlocks sheltered bays
that provide refuge from the relentless battering\nof the Southern 0cean.
In the depths, something stirs.
Humpback whales.
They have travelled over 5,000 miles\nto reach these waters.
The whales are harvesting krill,
shrimp-like creatures that begin to swarm here\nas soon as the ice retreats.
Diving into the heart of the swarm,\nteams of whales spiral round each other
in close coordination.
Now they turn, blasting air from their blowholes,\nand ascend towards their prey.
The krill becomes concentrated\nas the spiralling net of bubbles draws inwards.
The team of whales work round the clock\nfor the boom is short-lived.
Summer is already fading
and the whales will soon\nbe forced north as winter returns.
The sun's influence diminishes\nand the ocean starts to freeze.
The greatest seasonal change\non our planet is underway.
The ice grows at an extraordinary rate,\nadvancing two and a half miles a day.
In a matter of weeks,\nthe continent effectively doubles in size.
Life flees from Antarctica.
But one creature is just arriving.
Every winter, emperor penguins\nleave the comfort of their ocean home
and begin a remarkable journey.
They head towards their breeding grounds,\nalmost 1 00 miles inland.
Eventually, the emperor penguins\nreach the place where they were hatched,
an area sheltered by icebergs,\ntrapped in the frozen ocean.
Here they will raise the next generation.
But first, each must find a mate.
The males begin to serenade.
And if a female replies, they pair up,\nposing like statues.
New couples quickly form a strong bond.
They seem oblivious\nto the noisy crowd around them.
To cement their relationship,\nthe male steps out with his female.
The brief courtship complete,\nthere isn't a moment to lose.
With so much pressure to perform,\nany male would struggle to stay on top.
Several weeks later, and it seems\nthat most couplings were successful.
But producing the egg has taken its toll.
The females no longer\nhave the energy to incubate.
The male takes over.
It's still minus 20 degrees centigrade,
so the transfer must be done quickly\nor else the egg will freeze.
With no bare rock to nest on,\nthe male tucks the egg into a special pouch,
where he can keep it warm.
It requires an extraordinary piece of teamwork.
Driven by hunger, the exhausted females\nnow return to the ocean on their own,
repeating the epic journey they made\nwith the males only a month before.
Now the sun barely appears above the horizon.
As the days shorten,\nits warmth is withdrawn from the continent.
With the females gone,\nthe colony undergoes a strange transformation.
The males shuffle into groups,
their eggs still tucked away above their feet.
They lock together in tightly-packed huddles\nas they struggle to keep warm.
Speeding up the action reveals\nhow these huddles constantly shift and change
as each penguin works its way\ntowards the warmer interior.
Crammed into this scrum,\nthe birds are remarkably good-natured.
But they have to be.
If the huddle breaks, even for a moment,\nprecious heat escapes.
It's imperative they reform as quickly as possible,
for only by acting as one can the males\nwithstand the elements and protect their eggs.
But their greatest test lies ahead.
As winter advances,\nfrequent blizzards drive the temperature down.
It's now 60 degrees below zero.
The birds at the edge of the huddle\nbear the brunt of the 1 00-mile-an-hour winds
and so provide shelter\nto those taking their turn in the middle.
Abandoned by the sun,\nthe males are left alone with their eggs
to face the coldest, darkest winter on Earth.
At the northern extreme of our planet,\nthe sun rises for the first time in months,
illuminating a very different ice world.
Unlike Antarctica,\nthe Arctic is a vast frozen sea surrounded by land.
Here winter is coming to an end
but this bleak wilderness remains locked in ice.
Eider ducks break the silence.
They have stayed here,\nbraving the northern winter,
instead of flying south to warmer climes.
Flocks 40,000 strong\nsweep across the frozen wastes.
They all have the same goal, a polynya,
a permanent hole in the sea ice,
kept open throughout the winter\nby strong ocean currents.
This unusual duck pond\nprovides an overnight sanctuary
and, when day breaks, a rare chance to feed.
Just 1 0 metres beneath the ice,\nthe sea floor is carpeted with dense mussel beds.
These can only be reached\nduring a brief lull in the currents.
The ducks must quickly prise the mussels free\nbefore the tide starts to turn.
The window of opportunity is short.
As the current begins to build,\nit's up, up and away.
These permanent holes in the ice\nprovide seafood throughout the winter.
The diners attract others.
In the Arctic, any breach in the icy barrier\ncan be a lifeline.
Musk oxen create their own.
These giants have the strength\nto smash through the frozen crust
to graze on the vegetation below.
The ice-breakers create an opening\nfor other over-winterers.
Flocks of ptarmigan make unusual\ngrazing companions for the musk oxen,
whose entourage grows throughout the day.
This odd assembly of vegetarians\ndoesn't go unnoticed.
An arctic fox.
The musk oxen have recently given birth.
For the fox, it's a chance to scavenge.
But half a ton of mad hairy cow\nis not to be trifled with.
The calves are born well before the spring melt,
giving them a head start\nwhen summer finally arrives.
It must get to grips with its new ice world,
benign one minute, life-threatening the next.
Even in spring, winds chill to the bone.
The calf must stay close to its mother\nto avoid getting lost in the sudden blizzard.
Arctic wolves!
In the whiteout,\nthe threat is almost impossible to detect,
but the musk oxen\ninstinctively retreat to higher ground.
Forming a defensive ring around their calves,
the adults present a barricade\nthat few hunters could breach.
But the wolves need not risk injury today.
A calf has been left behind in the panic.
(H0WLING)
With each passing day,\nthe sun climbs higher in the sky
and its rays strike the Arctic more directly.
It's spring and new life stirs.
The polar bear cubs emerge from the den\nin which they were born.
Their mother stretches her legs\nafter five months under the snow.
They are just two months old\nand instinctively follow her lead.
A steep slope makes the best site for a den,\nbut it's a tricky place to take your first steps.
It may look like fun\nbut this is serious training for the task ahead.
There is no food on the slopes
and the family will need to head out\nacross the frozen sea
before the mother's milk runs dry.
Two weeks later, they are ready.
0ut on the sea ice, the female can hunt for seals
but it will take all her mothering skills\nto keep her cubs safe
in this dangerous world of ice.
The annual melt has begun.
This is a challenging time for the bear family.
0ne out of every two cubs\ndo not survive their first year out on the ice.
As the sun's influence increases,\nthe sea ice seems to take on a life of its own.
Glacial melt waters pour from the land,\nmingling with the sea
and speeding up the thaw.
The seascape is in constant flux
as broken ice is moved on by winds and currents.
The ice is becoming too weak\nto support a male polar bear.
He attempts to spread his weight,
but the ice that has supported him all winter\nis rapidly disintegrating.
Each year, as the climate warms,\nthe Arctic holds less ice.
This is a disaster for polar bears.
Without its solid platform,
they can't hunt the seals\nthey need in order to survive.
This may be a glimpse of the unstable future
faced by this magnificent creature.
As the ice disappears,\nseabirds return to the high Arctic.
Little auks arrive in their millions.
In some ways,\nthese birds are the penguins of the north.
They seek bare rock on which to lay their eggs\nand they look rather like penguins, too.
Unlike Antarctica, the Arctic\ncan be reached by land-based predators,
which is why little auks\nhave kept the ability to fly.
They use scree slopes to protect their eggs,\nburrowing up to a metre beneath the rocks.
At the height of summer, the sun never sets,
but just skims the horizon before rising again.
Migrants return to the Arctic from far and wide.
They've come to make the most of the brief flush\nof food and to produce their young.
Sandhill cranes have travelled\nall the way from New Mexico.
Their chicks join the growing band\nof youngsters exploring the tundra.
For a few months each year, the Arctic becomes\n''The land of the midnight sun''
and 24 hours of daylight\nallow animals to feed around the clock.
The arctic fox finally has enough food\nto raise her large family.
If you choose to nest in the open,\nyou must be prepared for a fight.
Arctic skuas will see off any trespassers,\neven large vegetarians.
The male polar bear's ice world\nhas finally vanished beneath him.
While the female is still kept on land\nby her dependent cubs,
the male can take to the sea in search of food.
Ducking and diving, he hopes to ambush seals
resting on the remaining fragments of ice.
In these new surroundings,\nhe is a surprisingly adept swimmer.
0nce an extremely rare sight,
polar bears have recently been seen\nover 60 miles from the shore.
There is now no turning back for this bear.
He is forced to head out into deeper water.
His giant front paws help him\nto fight the ocean currents.
He seems at home in the sea,\nbut he cannot swim indefinitely.
He will drown if he doesn't find land\nsomewhere in this vast ocean.
Walruses are now gathering on low-lying islands.
They gave birth on the sea ice,\nbut with this platform now gone,
they need a new place\nto haul out and nurse their young.
After several days at sea,\nthe male bear finally makes landfall,
drawn by the pungent smells\nemanating from the island.
By the end of the summer,\nthe bear has lost half his weight.
With the ice long gone,\nhe is forced onto land in search of food.
There will be no easy meals on this island.
Walruses are the largest seals in the world.
They weigh over a ton\nand are armed with tusks a metre long.
Exhausted from his swim,\nthe bear must regain his strength.
The next day, a sea fog shrouds the island.
The walruses sense that they are in danger.
Using the fog as cover,\nthe bear approaches the herd.
The adults close ranks around their young,
presenting a wall of blubber and hide.
He tests the barrier, but it stands firm.
It appears that the world's largest land carnivore\nhas met his match.
There must be a chink in the armour somewhere.
Not here!
This female walrus is shielding her pup,\nif he can just prise her off...
The bear's claws and teeth\ncan't penetrate her thick hide.
With the herd retreating to water,\nthe bear must move quickly.
Having failed with one,\nhe heads straight for another.
The chance of his first meal in months\nis slipping away.
He seems increasingly desperate.
It's now or never.
He must avoid the stabbing tusks if he is to win.
The flailing walrus is immensely powerful
and drags the bear away from the shallows\ntowards the safety of the herd.
It slips from his grasp.
0nly at the height of summer,
when bears are on the verge of starvation,\nwill they risk attacking such dangerous prey.
It was a gamble that this bear took and lost.
The stab wounds he received from the walrus\nare so severe that he can barely walk.
The walruses are calm again,
seemingly aware that the injured bear\nno longer poses a threat to them or their young.
Unable to feed, this bear will not survive.
If the global climate continues to warm,\nand the Arctic ice melts sooner each year,
it is certain that more bears will share this fate.
At the southern end of our planet,\nfiery ribbons are illuminating the winter skies.
The aurora australis.
This light brings no warmth to the male penguins,
who are still huddling,\ndefying the coldest conditions on the planet.
Their ordeal is drawing to a close.
Thirty days after it last set,\nthe sun rises once more on Antarctica.
Their appalling trials have all been for this.
Each father has just one meal left inside him.
He has been saving it all winter.
This single feed will sustain the chicks\nfor a few more days,
but the males have not eaten\nfor nearly four months.
If they do not eat soon,\nthey and their chicks will die.
But there is hope on the horizon.
The females are returning\nand their bellies are full with fish.
As they approach, waves of excitement\nripple through the huddle.
Each female calls to her mate\nand he, recognising her song, trumpets back.
Reunited at last.
The mother sees her chick for the first time.
She's keen to start parenting,
but the father needs persuading to surrender\nthe chick he has been caring for all winter.
He must now put his chick at risk.
In these temperatures it could freeze in seconds.
The male will have to let go.
Eventually, the transfer to the mother\nis safely made.
The chicks grow quickly\non a diet of fish and squid.
Soon they are keen to explore,\nbut always with mother in tow.
This chick is less fortunate,\nits mother has not returned to claim it.
Another orphan is searching for a new family,
but this female already has a chick of her own.
Some orphans receive too much mothering
from penguins whose own chicks\nhave not survived.
The urge to parent is so strong\nthat they will compete with one another
to adopt any chick they find.
Many of these squabbles end in tragedy\nas the poor chick is trampled to death.
Those chicks that do have parents\nquickly learn survival skills.
Even in spring,\nthey must huddle together for warmth,
just as their fathers did in the depths of winter.
A group of chicks has got lost in the blizzard.
Cold and disorientated, they search for the colony.
It will not be long\nbefore the storm claims its first victims.
By early summer,\nthe chicks are surprisingly well-developed
and now look ready to take on the world.
Those that survive their first year\nhave the best possible start in life
thanks to the extraordinary hardships\nendured by their parents.
Parents who battled\nwith the Antarctic winter and won.
In the Arctic, the two polar bear cubs\nare now independent of their mother
and they briefly reunite\nwhere their home ranges overlap.
Their time together will be fleeting.
Most of their lives are now spent alone,\nwandering the vast tracts of frozen ocean.
Following their mother has prepared them\nfor life at the pole,
an ever-changing land ruled by ice.
Whether they are ready for the bigger changes
that have begun to shape the ice worlds\nof our planet remains to be seen.