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Both poles of our planet are covered with ice.

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They're the largest
and most demanding wildernesses of all.

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Nowhere else on Earth
is seasonal change so extreme.

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It causes the ice
to advance and retreat every year.

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And all life here is governed by that.

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When the first polar explorers headed south,

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giant cathedrals of ice marked
their entry into uncharted territory.

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Passing the towering spires,

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they must have wondered
what unearthly sights lay in store.

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As they battled on,
the ice became increasingly dominant,

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but nothing could have prepared them
for the ice world that finally loomed into view.

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Terra incognita, "the unknown land".

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At the southernmost extreme of our planet,

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the continent of Antarctica is as large
as the United States of America.

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Ninety per cent of all the world's ice is found here.

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This frozen world is largely deserted
until the start of spring.

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Adélie penguins in a hurry.

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The clock is ticking.

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Instead of waiting for the summer melt,
the new arrivals hasten south over the frozen sea.

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They have come here to breed,
but polar summers are so short,

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they must be in position before the thaw starts.

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As the sea ice retreats,
life can journey farther south.

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Antarctic waters are so rich that visitors come
from far and wide to harvest them.

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Vast numbers of chinstrap penguins
come ashore to breed.

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No bird will lay their eggs directly onto ice

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so bare rock is a vital commodity.

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The best patches are worth the climb.

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The cliff tops are soon stained pink

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with the droppings of tens of thousands
of nesting penguins.

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Only in a land almost entirely covered in ice
could bare rock be reckoned an oasis.

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Some will travel
into the heart of the continent to find it.

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These are nunataks,
the exposed peaks of vast mountain ranges

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buried in ice over a mile deep.

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The eerie silence here is only broken in spring.

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The snow petrels have arrived and are courting.

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Antarctic petrels now join
the most southerly bird colony on Earth.

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The birds have flown inland
for over 300 miles to reach this breeding site.

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Once their eggs have hatched, they will be forced
repeatedly to make the 600-mile round trip

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to gather food in the ocean.

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First, though, valuable nesting places
must be defended from property thieves.

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After laying their eggs,
the petrels take time out to clean their plumage.

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The south polar skua is a formidable opportunist.

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But the skuas have not chanced upon the petrels,

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they've been waiting for them.

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These birds do not need to go
to the ocean for their food.

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The skuas can survive further south
than any other predator

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by exploiting the petrels' desperate need
for bare rock.

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Even at the height of summer, less than
three per cent of Antarctica is free of ice,

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and nearly all of that exposed rock
is found in one place,

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the Antarctic Peninsula.

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Its long arm extends further north
than the rest of the continent,

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so spring arrives here first.

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The thaw unlocks sheltered bays

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that provide refuge from the relentless battering
of the Southern Ocean.

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In the depths, something stirs.

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Humpback whales.

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They have travelled over 5, 000 miles
to reach these waters.

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The whales are harvesting krill,

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shrimp-like creatures that begin to swarm here
as soon as the ice retreats.

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Diving into the heart of the swarm,
teams of whales spiral round each other

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in close coordination.

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Now they turn, blasting air from their blowholes,
and ascend towards their prey.

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The krill becomes concentrated
as the spiralling net of bubbles draws inwards.

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The team of whales work round the clock
for the boom is short-lived.

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Summer is already fading

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and the whales will soon
be forced north as winter returns.

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The sun's influence diminishes
and the ocean starts to freeze.

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The greatest seasonal change
on our planet is underway.

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The ice grows at an extraordinary rate,
advancing two and a half miles a day.

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In a matter of weeks,
the continent effectively doubles in size.

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Life flees from Antarctica.

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But one creature is just arriving.

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Every winter, emperor penguins
leave the comfort of their ocean home

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and begin a remarkable journey.

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They head towards their breeding grounds,
almost 100 miles inland.

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Eventually, the emperor penguins
reach the place where they were hatched,

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an area sheltered by icebergs,
trapped in the frozen ocean.

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Here they will raise the next generation.

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But first, each must find a mate.

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The males begin to serenade.

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And if a female replies, they pair up,
posing like statues.

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New couples quickly form a strong bond.

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They seem oblivious
to the noisy crowd around them.

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To cement their relationship,
the male steps out with his female.

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The brief courtship complete,
there isn't a moment to lose.

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With so much pressure to perform,
any male would struggle to stay on top.

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Several weeks later, and it seems
that most couplings were successful.

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But producing the egg has taken its toll.

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The females no longer
have the energy to incubate.

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The male takes over.

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It's still minus 20 degrees centigrade,

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so the transfer must be done quickly
or else the egg will freeze.

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With no bare rock to nest on,
the male tucks the egg into a special pouch,

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where he can keep it warm.

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It requires an extraordinary piece of teamwork.

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Driven by hunger, the exhausted females
now return to the ocean on their own,

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repeating the epic journey they made
with the males only a month before.

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Now the sun barely appears above the horizon.

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As the days shorten,
its warmth is withdrawn from the continent.

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With the females gone,
the colony undergoes a strange transformation.

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The males shuffle into groups,

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their eggs still tucked away above their feet.

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They lock together in tightly-packed huddles
as they struggle to keep warm.

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Speeding up the action reveals
how these huddles constantly shift and change

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as each penguin works its way
towards the warmer interior.

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Crammed into this scrum,
the birds are remarkably good-natured.

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But they have to be.

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If the huddle breaks, even for a moment,
precious heat escapes.

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It's imperative they reform as quickly as possible,

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for only by acting as one can the males
withstand the elements and protect their eggs.

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But their greatest test lies ahead.

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As winter advances,
frequent blizzards drive the temperature down.

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It's now 60 degrees below zero.

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The birds at the edge of the huddle
bear the brunt of the 100-mile-an-hour winds

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and so provide shelter
to those taking their turn in the middle.

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Abandoned by the sun,
the males are left alone with their eggs

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to face the coldest, darkest winter on Earth.

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At the northern extreme of our planet,
the sun rises for the first time in months,

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illuminating a very different ice world.

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Unlike Antarctica,
the Arctic is a vast frozen sea surrounded by land.

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Here winter is coming to an end

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but this bleak wilderness remains locked in ice.

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Eider ducks break the silence.

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They have stayed here,
braving the northern winter,

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instead of flying south to warmer climes.

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Flocks 40, 000 strong
sweep across the frozen wastes.

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They all have the same goal, a polynya,

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a permanent hole in the sea ice,

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kept open throughout the winter
by strong ocean currents.

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This unusual duck pond
provides an overnight sanctuary

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and, when day breaks, a rare chance to feed.

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Just 10 metres beneath the ice,
the sea floor is carpeted with dense mussel beds.

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These can only be reached
during a brief lull in the currents.

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The ducks must quickly prise the mussels free
before the tide starts to turn.

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The window of opportunity is short.

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As the current begins to build,
it's up, up and away.

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These permanent holes in the ice
provide seafood throughout the winter.

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The diners attract others.

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In the Arctic, any breach in the icy barrier
can be a lifeline.

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Musk oxen create their own.

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These giants have the strength
to smash through the frozen crust

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to graze on the vegetation below.

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The ice-breakers create an opening
for other over-winterers.

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Flocks of ptarmigan make unusual
grazing companions for the musk oxen,

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whose entourage grows throughout the day.

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This odd assembly of vegetarians
doesn't go unnoticed.

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An arctic fox.

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The musk oxen have recently given birth.

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For the fox, it's a chance to scavenge.

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But half a ton of mad hairy cow
is not to be trifled with.

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The calves are born well before the spring melt,

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giving them a head start
when summer finally arrives.

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It must get to grips with its new ice world,

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benign one minute, life-threatening the next.

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Even in spring, winds chill to the bone.

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The calf must stay close to its mother
to avoid getting lost in the sudden blizzard.

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Arctic wolves!

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In the whiteout,
the threat is almost impossible to detect,

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but the musk oxen
instinctively retreat to higher ground.

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Forming a defensive ring around their calves,

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the adults present a barricade
that few hunters could breach.

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But the wolves need not risk injury today.

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A calf has been left behind in the panic.

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With each passing day,
the sun climbs higher in the sky

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and its rays strike the Arctic more directly.

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It's spring and new life stirs.

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The polar bear cubs emerge from the den
in which they were born.

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Their mother stretches her legs
after five months under the snow.

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They are just two months old
and instinctively follow her lead.

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A steep slope makes the best site for a den,
but it's a tricky place to take your first steps.

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It may look like fun
but this is serious training for the task ahead.

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There is no food on the slopes

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and the family will need to head out
across the frozen sea

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before the mother's milk runs dry.

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Two weeks later, they are ready.

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Out on the sea ice, the female can hunt for seals

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but it will take all her mothering skills
to keep her cubs safe

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in this dangerous world of ice.

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The annual melt has begun.

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This is a challenging time for the bear family.

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One out of every two cubs
do not survive their first year out on the ice.

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As the sun's influence increases,
the sea ice seems to take on a life of its own.

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Glacial melt waters pour from the land,
mingling with the sea

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and speeding up the thaw.

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The seascape is in constant flux

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as broken ice is moved on by winds and currents.

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The ice is becoming too weak
to support a male polar bear.

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He attempts to spread his weight,

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but the ice that has supported him all winter
is rapidly disintegrating.

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Each year, as the climate warms,
the Arctic holds less ice.

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This is a disaster for polar bears.

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Without its solid platform,

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they can't hunt the seals
they need in order to survive.

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This may be a glimpse of the unstable future

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faced by this magnificent creature.

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As the ice disappears,
seabirds return to the high Arctic.

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Little auks arrive in their millions.

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In some ways,
these birds are the penguins of the north.

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They seek bare rock on which to lay their eggs
and they look rather like penguins, too.

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Unlike Antarctica, the Arctic
can be reached by land-based predators,

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which is why little auks
have kept the ability to fly.

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They use scree slopes to protect their eggs,
burrowing up to a metre beneath the rocks.

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At the height of summer, the sun never sets,

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but just skims the horizon before rising again.

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Migrants return to the Arctic from far and wide.

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00:28:33,320 --> 00:28:38,310
They've come to make the most of the brief flush
of food and to produce their young.

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00:28:40,280 --> 00:28:44,270
Sandhill cranes have travelled
all the way from New Mexico.

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Their chicks join the growing band
of youngsters exploring the tundra.

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For a few months each year, the Arctic becomes
"The land of the midnight sun"

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and 24 hours of daylight
allow animals to feed around the clock.

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The arctic fox finally has enough food
to raise her large family.

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If you choose to nest in the open,
you must be prepared for a fight.

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Arctic skuas will see off any trespassers,
even large vegetarians.

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The male polar bear's ice world
has finally vanished beneath him.

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While the female is still kept on land
by her dependent cubs,

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the male can take to the sea in search of food.

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Ducking and diving, he hopes to ambush seals

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resting on the remaining fragments of ice.

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In these new surroundings,
he is a surprisingly adept swimmer.

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Once an extremely rare sight,

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polar bears have recently been seen
over 60 miles from the shore.

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There is now no turning back for this bear.

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He is forced to head out into deeper water.

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His giant front paws help him
to fight the ocean currents.

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He seems at home in the sea,
but he cannot swim indefinitely.

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He will drown if he doesn't find land
somewhere in this vast ocean.

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Walruses are now gathering on low-lying islands.

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They gave birth on the sea ice,
but with this platform now gone,

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they need a new place
to haul out and nurse their young.

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After several days at sea,
the male bear finally makes landfall,

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drawn by the pungent smells
emanating from the island.

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By the end of the summer,
the bear has lost half his weight.

233
00:34:12,840 --> 00:34:17,790
With the ice long gone,
he is forced onto land in search of food.

234
00:34:27,040 --> 00:34:30,550
There will be no easy meals on this island.

235
00:34:30,640 --> 00:34:33,830
Walruses are the largest seals in the world.

236
00:34:33,920 --> 00:34:38,790
They weigh over a ton
and are armed with tusks a metre long.

237
00:34:41,560 --> 00:34:46,190
Exhausted from his swim,
the bear must regain his strength.

238
00:34:53,120 --> 00:34:57,110
The next day, a sea fog shrouds the island.

239
00:34:57,200 --> 00:35:00,150
The walruses sense that they are in danger.

240
00:35:03,120 --> 00:35:07,190
Using the fog as cover,
the bear approaches the herd.

241
00:35:12,320 --> 00:35:14,990
The adults close ranks around their young,

242
00:35:15,080 --> 00:35:18,390
presenting a wall of blubber and hide.

243
00:35:25,600 --> 00:35:30,270
He tests the barrier, but it stands firm.

244
00:35:31,680 --> 00:35:36,470
It appears that the world's largest land carnivore
has met his match.

245
00:35:44,320 --> 00:35:47,470
There must be a chink in the armour somewhere.

246
00:35:50,920 --> 00:35:52,310
Not here!

247
00:36:00,920 --> 00:36:06,230
This female walrus is shielding her pup,
if he can just prise her off...

248
00:36:13,280 --> 00:36:17,670
The bear's claws and teeth
can't penetrate her thick hide.

249
00:36:26,280 --> 00:36:30,390
With the herd retreating to water,
the bear must move quickly.

250
00:36:35,680 --> 00:36:39,230
Having failed with one,
he heads straight for another.

251
00:36:56,320 --> 00:37:00,710
The chance of his first meal in months
is slipping away.

252
00:37:07,520 --> 00:37:09,910
He seems increasingly desperate.

253
00:37:15,520 --> 00:37:17,070
It's now or never.

254
00:37:18,440 --> 00:37:22,230
He must avoid the stabbing tusks if he is to win.

255
00:37:29,040 --> 00:37:32,030
The flailing walrus is immensely powerful

256
00:37:32,160 --> 00:37:36,590
and drags the bear away from the shallows
towards the safety of the herd.

257
00:37:52,080 --> 00:37:54,070
It slips from his grasp.

258
00:38:02,720 --> 00:38:04,950
Only at the height of summer,

259
00:38:05,080 --> 00:38:10,470
when bears are on the verge of starvation,
will they risk attacking such dangerous prey.

260
00:38:15,240 --> 00:38:18,390
It was a gamble that this bear took and lost.

261
00:38:21,800 --> 00:38:28,070
The stab wounds he received from the walrus
are so severe that he can barely walk.

262
00:38:32,960 --> 00:38:35,590
The walruses are calm again,

263
00:38:35,720 --> 00:38:41,350
seemingly aware that the injured bear
no longer poses a threat to them or their young.

264
00:38:57,200 --> 00:39:01,510
Unable to feed, this bear will not survive.

265
00:39:13,760 --> 00:39:19,990
If the global climate continues to warm,
and the Arctic ice melts sooner each year,

266
00:39:20,080 --> 00:39:23,910
it is certain that more bears will share this fate.

267
00:39:45,480 --> 00:39:51,910
At the southern end of our planet,
fiery ribbons are illuminating the winter skies.

268
00:39:52,000 --> 00:39:54,350
The aurora australis.

269
00:39:57,760 --> 00:40:01,830
This light brings no warmth to the male penguins,

270
00:40:01,960 --> 00:40:06,470
who are still huddling,
defying the coldest conditions on the planet.

271
00:40:20,320 --> 00:40:23,310
Their ordeal is drawing to a close.

272
00:40:31,200 --> 00:40:37,270
Thirty days after it last set,
the sun rises once more on Antarctica.

273
00:40:59,360 --> 00:41:03,510
Their appalling trials have all been for this.

274
00:41:19,280 --> 00:41:23,430
Each father has just one meal left inside him.

275
00:41:24,600 --> 00:41:27,550
He has been saving it all winter.

276
00:41:28,360 --> 00:41:33,910
This single feed will sustain the chicks
for a few more days,

277
00:41:34,000 --> 00:41:36,950
but the males have not eaten
for nearly four months.

278
00:41:37,040 --> 00:41:41,350
If they do not eat soon,
they and their chicks will die.

279
00:41:51,520 --> 00:41:55,030
But there is hope on the horizon.

280
00:42:06,160 --> 00:42:11,230
The females are returning
and their bellies are full with fish.

281
00:42:17,680 --> 00:42:21,630
As they approach, waves of excitement
ripple through the huddle.

282
00:42:30,200 --> 00:42:37,150
Each female calls to her mate
and he, recognising her song, trumpets back.

283
00:42:40,000 --> 00:42:42,470
Reunited at last.

284
00:42:45,800 --> 00:42:48,750
The mother sees her chick for the first time.

285
00:42:53,200 --> 00:42:56,070
She's keen to start parenting,

286
00:42:56,200 --> 00:43:01,350
but the father needs persuading to surrender
the chick he has been caring for all winter.

287
00:43:06,000 --> 00:43:08,710
He must now put his chick at risk.

288
00:43:08,800 --> 00:43:11,950
In these temperatures it could freeze in seconds.

289
00:43:15,920 --> 00:43:18,270
The male will have to let go.

290
00:43:25,000 --> 00:43:28,790
Eventually, the transfer to the mother
is safely made.

291
00:43:43,600 --> 00:43:47,670
The chicks grow quickly
on a diet of fish and squid.

292
00:44:01,640 --> 00:44:06,590
Soon they are keen to explore,
but always with mother in tow.

293
00:44:09,200 --> 00:44:14,430
This chick is less fortunate,
its mother has not returned to claim it.

294
00:44:16,840 --> 00:44:19,710
Another orphan is searching for a new family,

295
00:44:19,800 --> 00:44:23,270
but this female already has a chick of her own.

296
00:44:31,600 --> 00:44:34,230
Some orphans receive too much mothering

297
00:44:34,320 --> 00:44:37,550
from penguins whose own chicks
have not survived.

298
00:44:37,640 --> 00:44:41,790
The urge to parent is so strong
that they will compete with one another

299
00:44:41,880 --> 00:44:43,990
to adopt any chick they find.

300
00:44:49,080 --> 00:44:55,430
Many of these squabbles end in tragedy
as the poor chick is trampled to death.

301
00:45:07,240 --> 00:45:11,950
Those chicks that do have parents
quickly learn survival skills.

302
00:45:14,040 --> 00:45:17,150
Even in spring,
they must huddle together for warmth,

303
00:45:17,240 --> 00:45:20,870
just as their fathers did in the depths of winter.

304
00:45:24,520 --> 00:45:27,430
A group of chicks has got lost in the blizzard.

305
00:45:34,400 --> 00:45:38,150
Cold and disorientated, they search for the colony.

306
00:45:44,880 --> 00:45:49,750
It will not be long
before the storm claims its first victims.

307
00:46:07,920 --> 00:46:12,910
By early summer,
the chicks are surprisingly well-developed

308
00:46:13,000 --> 00:46:16,470
and now look ready to take on the world.

309
00:46:17,960 --> 00:46:22,590
Those that survive their first year
have the best possible start in life

310
00:46:22,680 --> 00:46:27,230
thanks to the extraordinary hardships
endured by their parents.

311
00:46:28,200 --> 00:46:33,510
Parents who battled
with the Antarctic winter and won.

312
00:46:42,920 --> 00:46:48,070
In the Arctic, the two polar bear cubs
are now independent of their mother

313
00:46:48,160 --> 00:46:51,830
and they briefly reunite
where their home ranges overlap.

314
00:46:59,200 --> 00:47:02,310
Their time together will be fleeting.

315
00:47:02,400 --> 00:47:08,910
Most of their lives are now spent alone,
wandering the vast tracts of frozen ocean.

316
00:47:37,160 --> 00:47:41,670
Following their mother has prepared them
for life at the pole,

317
00:47:41,760 --> 00:47:45,590
an ever-changing land ruled by ice.

318
00:47:47,640 --> 00:47:49,750
Whether they are ready for the bigger changes

319
00:47:49,840 --> 00:47:55,190
that have begun to shape the ice worlds
of our planet remains to be seen.