The polar winter. This is the planet at its most hostile. Those that stay here at this time must face the harshest conditions on Earth. The High Arctic in late autumn. Empty and abandoned to ice and snow. Most animals migrated south weeks ago. The few that stay will face violent winds, plunging temperatures and months of darkness. A female polar bear searches for shelter. Other bears are out on the frozen ocean looking for food, but she won't feed again until next spring. Using as little energy as possible, she starts to dig a shallow nest. The snow here is easy to work. It's soft and light. A sure sign that soon, plenty more will accumulate on this slope as winter advances. And that is what she needs. If she has chosen well, the Arctic wind will do much of her work for her. Once the snow here is deep enough, she will dig down to make a den. She'll then lie, waiting for her cubs to be born as winter sets in. The sea ice already covers twice the area it did in summer. Those animals that, a few weeks ago, came down here to feed in the sea, are now locked out. But there are windows in this white desert. Gateways to the rich ocean below. Most birds have migrated south. But in the frozen Bering Sea, ducks of one particular species are gathering together in a single, immense flock. Spectacled eiders. Hundreds of thousands of them. With the seas that fed them throughout the summer all but frozen, this is the eider's last refuge. Such patches of open water, kept free of ice by strong currents, are given a Russian name, polynyas. If this polynya stays open all winter, the ducks will have avoided an exhausting migration. It's a gamble and a tremendous risk, especially for the spectacled eider, for this polynya holds the world's entire population. Surrounded by ice, these spectacled eiders' survival depends on this single, vulnerable oasis. And conditions here can change very fast. A smaller pool. Here, common eider ducks are learning a bitter lesson. An early winter storm has caught them out and the ice is closing in. The pool is shrinking, and the ducks are freezing to death. These gamblers have lost. As the sun's influence continues to dwindle here in the north, the cold pushes south into Arctic lands. Winter brings a devastatingly-destructive force. Frost. Ice crystals form as moisture in the air freezes. Inside plants, the same thing is happening to the water in their cells and their sap. With these first frosts, most plants die. Their insides ripped apart by ice crystals. Coniferous trees, however, like fir and pine, can withstand very low temperatures. It's these trees that create the greatest forest on our planet. This is the Taiga. It encircles the globe and contains one third of all the trees on earth. As winter deepens, frigid air from the Arctic meets warm, moist air from the south, producing particularly heavy snows. For the big animals of the forest, there's no way of hiding from the winter. But size, for the Bison of Northern Canada, is, in fact, their salvation. Big bodies lose less heat and can carry more insulation. That may be why these bison are the largest land animals in North America. And only a large predator can tackle them. Wolves hunt better in packs, but there are only two of them here and the prey they pursue are giants. A vast Arctic wilderness stretches all around. In it somewhere, there are bison. These deep tracks in the snow are easy to follow, but which way were the bison going? They have detected a scent. The hunt is on. Running in the bison's tracks is easy. The snow here is compressed and firm. At this rate, the wolves will soon catch up. They attempt a shortcut. The giant bison can plough on through virgin snow, but for the wolves, deep snow is a hindrance, and now they're losing ground. The hunters rejoin the bison's tracks and the contest is on again. The bison are the only prey here in winter. The wolves have no option, they must tackle them. But bison are ten times their size. In winter, the line between life and death is so narrow that for a wolf, even a small injury could be fatal. It's crucial they select the right target. This one is very big. Good decision. This is smaller, a yearling. Even though it's young, it's none the less heavier than both wolves combined and it is extremely powerful. The male wolf backs off. Perhaps he's frightened of injury. But the female is more determined, or more desperate. Having given so much already, she must make the kill. This is a battle of life and death for them both. The bison has no strength left. The battle is over. Winter deepens. The snow continues to accumulate, smothering the forest. Some trees are loaded with three tonnes of snow. It shuts out what little light there is at these latitudes, and that shortens the growing season still further and limits how far north trees are able to grow. So, snow influences both the shapes of the trees and the extent of this forest. And it affects the animals, too. SQUAWK! It helps to have friends if you want to find food at this time of the year. Wolverines and ravens, a match made in the Taiga. It's the hardest time of the year to find food. But the wolverine knows that a raven's call is as good as a dinner bell. And the raven has just recruited the best can opener in the forest. The moose carcass is frozen solid, but the wolverine has immensely powerful jaws, well able to deal with frozen food. The raven's bill is more suited to leftovers. The raven has to be patient. A wolverine's appetite is legendary. According to folklore, it can eat more at one sitting than any other creature in the forest, hence its other name, the glutton. What the wolverine can't eat now, it stores in the deep freeze. It will bury these food parcels across the forest, planning ahead for leaner times. For a few, the snow is an ally. Voles stay active throughout the winter beneath the snow, despite the freezing temperatures above. Travelling along tiny corridors, they move from pocket to pocket of perfectly refrigerated food. Snow is a great insulator, so down here, the temperature never falls more than a degree or so below zero. And that's warm enough for a vole to thrive, even if it lacks the stature of a bison. In some years, vole numbers boom, and then they are forced to break cover to search for more food. The great grey owl is a silent and very skilful hunter, but deep snow can be beyond its reach. But such snow is no barrier to one predator. The least weasel. A tiny hunter, and the vole's nemesis. Its body is exactly the same width as a vole's, so there's nowhere a vole can go that the weasel can't follow. INSTRUMENTAL The weasel's long, slender shape is perfect for hunting in tunnels, but the worst possible shape for staying warm. So they need a special way of doing that. She plucks the fur from its body tuft by tuft. And now she puts it all together to make a cosy blanket under which to sleep in her den below the snow. Midwinter in the northern forests. The sun is so low that it's twilight at midday. Further north in the high Arctic, the sun has been below the horizon for months and it'll be several more before it appears again. The male polar bear spends winter out on the frozen ocean. Few creatures can endure these conditions. If he's lucky, he will find a carcass that could provide a little food, possibly that of a fox or another polar bear. But otherwise, he must live on his reserves. This is the time to scrape by, to wait. But on lee-side slopes, beneath the snow, new lives are beginning. The cubs are born blind and tiny. An early birth is easier on the mother, who is barely awake. Despite her sleepiness, her instinct to nurse is overwhelming. The cub's clucking calls stimulate her to produce milk. And what milk! It's nine times richer than our own and enables her to double their weight every few weeks. It's over two months since the autumn snows first arrived. In two more months, polar bear families will emerge onto the snowy slopes all around the Arctic. But for now, they lie protected within their icy cocoons. A hundred miles above the earth, the Aurora lights up the sky. After travelling millions of miles across space, solar winds, attracted by the magnetic pull of the poles, collide with the earth's atmosphere. Trillions of charged particles dance across the sky. Above the Arctic, the Aurora Borealis - the Northern Lights. In the south, it's the Aurora Australis - the Southern lights, that bring light to Antarctica's long winter. These spectacular lightshows are only a tease. Solar energy maybe, but no warmth that will help the emperor penguins. The Arctic winter is brutally cold, but in Antarctica, the darkest months are even more savage. The male penguins have not eaten for months, and have only each other for protection from the gales. Each has been entrusted with a single precious egg balanced on the top of its feet. If the egg were to drop onto the ice, even for a moment, the chick inside would die and all this would count for nothing. The emperors are not entirely alone. The Weddell seal. The only mammal to remain here throughout the winter. They must have breathing holes, and so have to constantly scrape away the ice that threatens to close them, rasping away fresh build-ups with special wide-gaped jaws. Beneath the ice, they are beyond the reach of the bitter winter winds. A warm bath, compared to the conditions overhead. The roof of ice insulates this world from the wild fluctuations above. The temperature down here has barely changed Of course, animals must still be hardy. These borchgrevinki fish have antifreeze in their blood, so they are untroubled swimming among the ice crystals. Many animals here are remarkably long-lived, perhaps because the conditions are so stable. Some sponges could be a thousand years old and large enough for a human to hide in. There could hardly be a greater contrast to the bleak, windswept world just above. But there is a constant danger here. Swirling patterns in the water reveal its presence. They are made by brine. Super-concentrated salt water. It's a warning. New sea ice forming above leaves behind brine that is so extremely salty, it sinks rapidly. As it descends, the sea water around it freezes instantly and forms a sheath of ice, a brinicle, that grows downwards towards the seafloor. Winter is reaching down from the cold world above. As it touches the seafloor, it kills whatever living thing it contacts by encasing it in a tomb of ice. Even in the relative warmth of the water, the lethal cold of winter threatens life on the seafloor. Another more constant attack rises from below. Pressure in the depths keeps water liquid, even though its temperature is far from freezing. Currents bring up this colder water, but it turns to ice, covering everything that can't move away from it. And then, the ice, being lighter than water, begins to float, lifting away anything attached. Even some of the faster-moving animals are caught. So the ice cleanses the seafloor and strews the ceiling above with remnants of life from below. A sign that winter is nearly over. The songs of male Weddell seals challenging their rivals to battle. The male seal's calls He aims to control the best breathing holes for they will determine his mating rights in spring. His calls create powerful shockwaves in the water. They are threats. This hole has already been claimed, and the owner will not surrender it willingly. No change this time. The challenger will need to find a hole with a weaker owner. The battles continue until the females arrive, and that time is now close. The sun returns to Antarctica. The longest night on earth has ended and winter begins to give way to spring. Female emperor penguins. After four months feeding at sea, they are returning sleek and fat. Penguins, it seems, can fly after all. But a winter at sea has left them a little out of practice. There is no time to waste. Far away, the males are waiting. While the females were gone, the sea ice grew, and it's now twice the distance from its edge back to their colony. are in desperate need of help. Despite doing all they can to conserve their energy, many are close to dying from starvation. SQUAWK! SQUAWK! SQUAWKING Reunited after three months apart. The reward for her return? A first glimpse of her chick. The reunion made, it's time for the handover. The father finds it hard to let go. Some gentle persuasion is needed. He has endured the most appalling conditions on earth to ensure the survival of his chick, and the bond is strong. The exchange must be quick, as the tiny chick, unprotected, could freeze to death in seconds. A task that began in autumn has been completed. Despite the huge odds against it, the precious chick has survived the winter and is now with its mother. And she has food. The chick's first fresh meal. Her mission is complete. But for other mothers, the journey ends in disappointment. Their chicks have not survived. The females' need to nurture remains strong. Any chick that strays from its parent is at grave risk of being kidnapped. The chaos may be a consequence of frustrated parental urge, but the outcome can be tragic. For those emperor penguins that survive, the worst is over for this year. There will soon be abundant food for everyone. The emperors have taken on the polar winter and won. The gamble has paid off. All other animals escaped, only they remained to raise their young. And now, it is they who will now benefit most from the rich southern spring. Another arrival is the certain confirmation that spring is here. The Adelies, having spent winter at sea, have come back, but they have not even mated yet, let alone laid their eggs. Now, at last, the male emperors can return to where they are most at home. One season ends and another begins. The penguins will soon be joined by migrants and the far south will bustle with life for a few frantic months. But there are only a special few, north and south, who can survive winter at the farthest ends of our planet. Next time, Frozen Planet is with the people of the polar regions. From traditional ways of survival... ..to the very frontiers of science. More than any other season in the poles, winter was to pose the greatest challenge to the Frozen Planet crew. The team endured winds and temperatures as low as Their boats were trapped in sea ice for days, while bears trapped others indoors. Some of the coldest conditions experienced were near the Arctic circle in Northern Canada. In winter, temperatures drop On the edge of the vast Taiga forest, the team hoped to film one of the most remarkable predator-prey relationships on the planet. What they experienced was the struggle against the elements. What they witnessed was a more profound struggle for life and death. Wood Buffalo National Park The size of Denmark. One of the few cameramen to have filmed wolves and bison here is Jeff Turner. I first tried to film bison and wolves And getting around on the ground is incredibly difficult at any time of the year, but in the winter, it's tough. We quickly realised the only way we'd get anything here was we had to get up in the air. The Frozen Planet team had never attempted aerial filming in such low temperatures. The first challenge for director Chadden Hunter was protecting the sensitive aerial camera. short-circuit like fireworks. Oh! Ooo! How cold can you fly this chopper? Aerial cameraman Michael Kelem comes from sunny California and has never experienced temperatures so low. What kind of temperatures can you operate down to? Fahrenheit in LA-kind of weather. Yeah? I'm only rated for Santa Monica. Operating delicate controls while wearing gloves is not easy. A computer and joystick are needed to control the aerial camera attached to the outside of the helicopter. Michael would have to choose between dexterity or warmth. INDISTINCT RADIO With their camera mounted on the nose, the aerial team could now fly hundreds of miles in search of the animals. Jeff, meanwhile, is still on foot, scouting the other side of the national park. 'I wonder what Jeff's getting on the ground?' Oh, man, it's cold! so...we've got to keep these heaters in here. Keep the camera running at these temperatures. Chadden catches a glimpse of the elusive wolf pack. Yeah. Come in. 'I can see wolves. I can see wolves near buffalo.' We've got a big pack of wolves here. The wolf pack turns out to number One of the largest ever filmed. 'There's two more walking in from the left.' It looks like they're already on the hunt. The helicopter allows the team to shadow the wolf pack without disturbing them. So, let's go back into that nice slow circle, move around them. 'They look pretty determined.' The pack are moving in on their prey, and the aerial team are perfectly placed. 'They're making their, definitely making their move. There's buffalo on the move.' The chase is on. Extreme concentration is now needed, both from the pilot and the cameraman to keep the shots smooth. 'Yeah, there's more coming in now. 'There's a nice tight little pack challenging them. 'We've got a nice shot on the pack, the wolves coming up at the back. 'You have the still? Yeah. I got them. 'I'm going to bring it up a bit. OK, OK, tighten up just a hair. 'I want to get two shots of the wolves at the back of the pack.' The wolf pack have picked out a young bison. 'Holy BLEEP! ' As Michael struggles to hold the shot steady, no-one is prepared for what came next. 'Oh! Hey, look, I just knocked him down, pretty much.' A one-ton bull charges through, taking out both the wolves and the young bison. I've never seen anything like that in the wild. That is unbelievable. The team have filmed extraordinary new behaviour from the air, but Jeff was nowhere nearby. To complete the sequence, they will need to work together. Only by using the helicopter to position Jeff will they be able to get both air and ground coverage. This pack will now feast for days, so the crew must move on. Jeff takes the team deeper into the wilderness in search of a new wolf pack. With everyone onboard, there are more eyes for spotting. Bison on the run can only mean one thing - wolves. But this time, only a pair. 'I got... No, there's a big...' Jeff must anticipate the animal's every move and direct their helicopter to where he needs to be dropped off. They're going this way. I had to figure out where the buffalo were going to run and try to get there ahead of them. If I got dropped off in the wrong position, we would blow it. There wasn't going to be a second chance. That moment, when you get out of the helicopter, and it leaves you behind, you feel an incredible sense of isolation. On the lake. I'll guide you in. Just down to the right. Down. Zoom in. Watch it. You're on the back guy. We're on the back guy. 'On the back. Which one is he going for?' I don't know. Oh, there's one broken off. Oh! Oh, my gosh! They're coming in now. When I saw the herd coming around the corner, I knew we'd picked the right spot. I was right in front of them. And the buffalo running right towards you, it definitely does get your heart pounding. The bison have poor eyesight and can't see Jeff. It's now a test of nerves. Weighing one ton and running one wrong step and the bison could kill Jeff. At the last second, they spot him and three peel away to the other side. At the back of the herd, the wolves have closed in. They're trying to separate that little one. Oh, they've got it! Both wolves are on him. Both clamping down on him. He got stomped there. What followed was one of the most emotional and powerful scenes the Frozen Planet team were ever to film. Jeff has picked his position perfectly. from the animals and able to film a truly epic battle. I had never been so close to a wolf and bison battle. I could hear them breathing. And the power of the buffalo, the way he just threw this wolf around like it was a rag doll. I couldn't believe the beating this wolf was taking. The struggles we'd had with the winter and the cold and the snow, just felt so insignificant compared to these two animals that were struggling for their very lives. For over an hour, I watched this wolf and bison battle each other to a standstill. They were both unbelievably exhausted. Watching these two animals engaged in this massive life-and-death struggle was one of the most powerful things I'd ever witnessed in the wild. As a film-maker, you're definitely torn watching something like this. You know you need to keep filming, and you know this is a natural event, but it's also incredibly sad to know that one of these two magnificent animals isn't going to make it. The bison is fatally wounded. The battle is over. The team have managed to capture a remarkable hunt from both ground and air. All of them have been humbled by witnessing an extraordinary winter struggle for survival. Subtitles by Red Bee Media Ltd E-mail subtitling@bbc.co.uk