Both poles of our planet are covered with ice.
They're the largest and most demanding wildernesses of all.
Nowhere else on Earth is seasonal change so extreme.
It causes the ice to 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 their entry into uncharted territory.
Passing the towering spires, they must have wondered what unearthly sights lay in store.
As they battled on, the ice became increasingly dominant, but nothing could have prepared them for 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 as the United States of America.
Ninety per cent of all the world's ice is found here.
This frozen world is largely deserted until the start of spring.
Adelie penguins in a hurry.
The clock is ticking.
Instead of waiting for the summer melt, the new arrivals hasten south over the frozen sea.
They have come here to breed, but polar summers are so short, they must be in position before the thaw starts.
As the sea ice retreats, life can journey farther south.
Antarctic waters are so rich that visitors come from far and wide to harvest them.
Vast numbers of chinstrap penguins come 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 of nesting penguins.
0nly in a land almost entirely covered in ice could bare rock be reckoned an oasis.
Some will travel into the heart of the continent to find it.
These are nunataks, the 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 the most southerly bird colony on Earth.
The birds have flown inland for over 300 miles to reach this breeding site.
0nce their eggs have hatched, they will be forced repeatedly to make the 600-mile round trip to gather food in the ocean.
First, though, valuable nesting places must be defended from property thieves.
After laying their eggs, the 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 to the ocean for their food.
The skuas can survive further south than any other predator by exploiting the petrels' desperate need for bare rock.
Even at the height of summer, less than three per cent of Antarctica is free of ice, and nearly all of that exposed rock is found in one place, the Antarctic Peninsula.
Its long arm extends further north than the rest of the continent, so spring arrives here first.
The thaw unlocks sheltered bays that provide refuge from the relentless battering of the Southern 0cean.
In the depths, something stirs.
Humpback whales.
They have travelled over 5,000 miles to reach these waters.
The whales are harvesting krill, shrimp-like creatures that begin to swarm here as soon as the ice retreats.
Diving into the heart of the swarm, teams of whales spiral round each other in close coordination.
Now they turn, blasting air from their blowholes, and ascend towards their prey.
The krill becomes concentrated as the spiralling net of bubbles draws inwards.
The team of whales work round the clock for the boom is short-lived.
Summer is already fading and the whales will soon be forced north as winter returns.
The sun's influence diminishes and the ocean starts to freeze.
The greatest seasonal change on our planet is underway.
The ice grows at an extraordinary rate, advancing two and a half miles a day.
In a matter of weeks, the continent effectively doubles in size.
Life flees from Antarctica.
But one creature is just arriving.
Every winter, emperor penguins leave the comfort of their ocean home and begin a remarkable journey.
They head towards their breeding grounds, almost 100 miles inland.
Eventually, the emperor penguins reach the place where they were hatched, an area sheltered by icebergs, trapped 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, posing like statues.
New couples quickly form a strong bond.
They seem oblivious to the noisy crowd around them.
To cement their relationship, the male steps out with his female.
The brief courtship complete, there isn't a moment to lose.
With so much pressure to perform, any male would struggle to stay on top.
Several weeks later, and it seems that most couplings were successful.
But producing the egg has taken its toll.
The females no longer have the energy to incubate.
The male takes over.
It's still minus 20 degrees centigrade, so the transfer must be done quickly or else the egg will freeze.
With no bare rock to nest on, the 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 now return to the ocean on their own, repeating the epic journey they made with the males only a month before.
Now the sun barely appears above the horizon.
As the days shorten, its warmth is withdrawn from the continent.
With the females gone, the 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 as they struggle to keep warm.
Speeding up the action reveals how these huddles constantly shift and change as each penguin works its way towards the warmer interior.
Crammed into this scrum, the birds are remarkably good-natured.
But they have to be.
If the huddle breaks, even for a moment, precious heat escapes.
It's imperative they reform as quickly as possible, for only by acting as one can the males withstand the elements and protect their eggs.
But their greatest test lies ahead.
As winter advances, frequent blizzards drive the temperature down.
It's now 60 degrees below zero.
The birds at the edge of the huddle bear the brunt of the 100-mile-an-hour winds and so provide shelter to those taking their turn in the middle.
Abandoned by the sun, the males are left alone with their eggs to face the coldest, darkest winter on Earth.
At the northern extreme of our planet, the sun rises for the first time in months, illuminating a very different ice world.
Unlike Antarctica, the 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, braving the northern winter, instead of flying south to warmer climes.
Flocks 40,000 strong sweep across the frozen wastes.
They all have the same goal, a polynya, a permanent hole in the sea ice, kept open throughout the winter by strong ocean currents.
This unusual duck pond provides an overnight sanctuary and, when day breaks, a rare chance to feed.
Just 10 metres beneath the ice, the sea floor is carpeted with dense mussel beds.
These can only be reached during a brief lull in the currents.
The ducks must quickly prise the mussels free before the tide starts to turn.
The window of opportunity is short.
As the current begins to build, it's up, up and away.
These permanent holes in the ice provide seafood throughout the winter.
The diners attract others.
In the Arctic, any breach in the icy barrier can be a lifeline.
Musk oxen create their own.
These giants have the strength to smash through the frozen crust to graze on the vegetation below.
The ice-breakers create an opening for other over-winterers.
Flocks of ptarmigan make unusual grazing companions for the musk oxen, whose entourage grows throughout the day.
This odd assembly of vegetarians doesn'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 is not to be trifled with.
The calves are born well before the spring melt, giving them a head start when 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 to avoid getting lost in the sudden blizzard.
Arctic wolves!
In the whiteout, the thr*at is almost impossible to detect, but the musk oxen instinctively retreat to higher ground.
Forming a defensive ring around their calves, the adults present a barricade that few hunters could breach.
But the wolves need not risk injury today.
A calf has been left behind in the panic.
With each passing day, the 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 in which they were born.
Their mother stretches her legs after five months under the snow.
They are just two months old and instinctively follow her lead.
A steep slope makes the best site for a den, but it's a tricky place to take your first steps.
It may look like fun but this is serious training for the task ahead.
There is no food on the slopes and the family will need to head out across 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 to 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 do not survive their first year out on the ice.
As the sun's influence increases, the sea ice seems to take on a life of its own.
Glacial melt waters pour from the land, mingling 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 to support a male polar bear.
He attempts to spread his weight, but the ice that has supported him all winter is rapidly disintegrating.
Each year, as the climate warms, the Arctic holds less ice.
This is a disaster for polar bears.
Without its solid platform, they can't hunt the seals they need in order to survive.
This may be a glimpse of the unstable future faced by this magnificent creature.
As the ice disappears, seabirds return to the high Arctic.
Little auks arrive in their millions.
In some ways, these birds are the penguins of the north.
They seek bare rock on which to lay their eggs and they look rather like penguins, too.
Unlike Antarctica, the Arctic can be reached by land-based predators, which is why little auks have kept the ability to fly.
They use scree slopes to protect their eggs, burrowing 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 of food and to produce their young.
Sandhill cranes have travelled all the way from New Mexico.
Their chicks join the growing band of youngsters exploring the tundra.
For a few months each year, the Arctic becomes ''The land of the midnight sun'' and 24 hours of daylight allow animals to feed around the clock.
The arctic fox finally has enough food to raise her large family.
If you choose to nest in the open, you must be prepared for a fight.
Arctic skuas will see off any trespassers, even large vegetarians.
The male polar bear's ice world has finally vanished beneath him.
While the female is still kept on land by 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, he is a surprisingly adept swimmer.
0nce an extremely rare sight, polar bears have recently been seen over 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 to fight the ocean currents.
He seems at home in the sea, but he cannot swim indefinitely.
He will drown if he doesn't find land somewhere in this vast ocean.
Walruses are now gathering on low-lying islands.
They gave birth on the sea ice, but with this platform now gone, they need a new place to haul out and nurse their young.
After several days at sea, the male bear finally makes landfall, drawn by the pungent smells emanating from the island.
By the end of the summer, the bear has lost half his weight.
With the ice long gone, he 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 and are armed with tusks a metre long.
Exhausted from his swim, the 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, the 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 has met his match.
There must be a chink in the armour somewhere.
Not here!
This female walrus is shielding her pup, if he can just prise her off...
The bear's claws and teeth can't penetrate her thick hide.
With the herd retreating to water, the bear must move quickly.
Having failed with one, he heads straight for another.
The chance of his first meal in months is 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 towards the safety of the herd.
It slips from his grasp.
0nly at the height of summer, when bears are on the verge of starvation, will they risk attacking such dangerous prey.
It was a gamble that this bear took and lost.
The s*ab wounds he received from the walrus are so severe that he can barely walk.
The walruses are calm again, seemingly aware that the injured bear no longer poses a thr*at to them or their young.
Unable to feed, this bear will not survive.
If the global climate continues to warm, and the Arctic ice melts sooner each year, it is certain that more bears will share this fate.
At the southern end of our planet, fiery ribbons are illuminating the winter skies.
The aurora australis.
This light brings no warmth to the male penguins, who are still huddling, defying the coldest conditions on the planet.
Their ordeal is drawing to a close.
Thirty days after it last set, the 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 for a few more days, but the males have not eaten for nearly four months.
If they do not eat soon, they and their chicks will die.
But there is hope on the horizon.
The females are returning and their bellies are full with fish.
As they approach, waves of excitement ripple through the huddle.
Each female calls to her mate and 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 the 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 is safely made.
The chicks grow quickly on a diet of fish and squid.
Soon they are keen to explore, but always with mother in tow.
This chick is less fortunate, its 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 have not survived.
The urge to parent is so strong that they will compete with one another to adopt any chick they find.
Many of these squabbles end in tragedy as the poor chick is trampled to death.
Those chicks that do have parents quickly learn survival skills.
Even in spring, they 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 before the storm claims its first victims.
By early summer, the chicks are surprisingly well-developed and now look ready to take on the world.
Those that survive their first year have the best possible start in life thanks to the extraordinary hardships endured by their parents.
Parents who battled with the Antarctic winter and won.
In the Arctic, the two polar bear cubs are now independent of their mother and they briefly reunite where their home ranges overlap.
Their time together will be fleeting.
Most of their lives are now spent alone, wandering the vast tracts of frozen ocean.
Following their mother has prepared them for 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 of our planet remains to be seen.