In Southeast Alaska, there's an ice-bound Eden that harbors possibly the richest temperate rainforests of all. Where the coastal mountains meet the Pacific, lays the Alexander Archipelago, a remote island chain running for almost 300 miles along the Alaskan Panhandle. This frozen frontier is one of the last great wildernesses of North America. And a stronghold for the country's highest diversity of megafauna, feasting on the abundance of fleeting summers to make it through relentless winters.

At the top of our planet lies a magical realm, the Arctic Ocean. After four months of winter darkness, the sun returns to reveal a frozen ocean covered in ice. Mother polar bears emerge from their hillside dens and lead their cubs down to the sea ice to hunt, while a young male and female bear forge a surprising friendship out on the ice.
For others, the frozen sea is a trap. A pod of beluga whales has been confined to an ice hole for five months, slowly starving to death as the food around them runs out. Their salvation lies in the strengthening sun that comes with spring, melting the sea ice, allowing their escape.
Off the east coast of Greenland, the floating pack ice in spring is a nursery ground for harp seals. Mothers and pups have just a few weeks together for the pup to learn to swim before she leaves him to fend for himself. But in today’s warming climate, storms can tip helpless youngsters into the sea before they are strong enough to fend for themselves.
Summer is a time of plenty in the Arctic Ocean as plankton blooms feed millions of tiny mouths, such as bizarre skeleton shrimps, as well as the biggest: bowhead whales. These ancient and long-lived whales arrive en masse every year at secret locations known as whale spas. But today, with the loss of summer sea ice, their peace is shattered by orcas from the south. These daring predators are bold enough to take on the much larger bowheads, targeting their vulnerable calves.
The 24-hour daylight of the Arctic summer attracts visitors from afar, including huge flocks of seabirds like crested auklets. A male must use both his song and a secret tangerine perfume if he is to attract a mate. For the resident walrus, the summer heat can be unbearable. After hauling himself to the beach to moult, an old male uses an ingenious technique to get himself back to the cool of the water - a roly-poly!
Summers in the Arctic today bring record-breaking heat. With climate change, it is warming faster than anywhere else on Earth. It is predicted that the Arctic Ocean could become ice-free each summer by 2035, raising new challenges for polar bears. Without sea ice, more and more bears are becoming stranded on remote Arctic islands. It's a dangerous place to be for a mother bear with cubs, surrounded by larger, predatory males.

Antarctica is the most hostile of all earth’s frozen worlds. Yet even here, amongst some of the most challenging conditions on the planet, life finds a way not just to survive, but thrive.
Our journey begins at the far edge of the continent, on its far-flung sub-Antarctic islands. Here we meet king penguins that, to feed at sea, must face the danger of ferocious leopard seals lurking in the shallows. On another island, we witness for the first time male Antipodean wandering albatross partnering up with each other as the females in their population are disappearing due to fishing activity.
Heading towards the continent of Antarctica, we traverse the roughest seas on earth - the Southern Ocean - where we find the rarely filmed Antarctic blue whale, the largest animal to have ever lived. At the edge of Antarctica, the sea is so cold that it freezes over, creating a vital ice platform for a mother Weddell seal to raise her precious pup. Still, she needs to protect him from aggressive males.
In spring, the coast of Antarctica is free of snow, drawing in thousands of breeding chinstrap penguins. Stones are at a premium to build their elevated nests and protect chicks from meltwater. But stealing is commonplace, and to make matters worse, with climate change we find chicks today shivering with hypothermia – a warming Antarctica means increased meltwater. Other residents are facing an uncertain future too, including wave-washing killer whales. We discover that their favourite prey, Weddell seals, are now harder to reach, so instead they are resorting to targeting much more feisty prey, including leopard seals, an apex predator in its own right. This dramatic encounter has never been filmed before.
Travelling into the interior of the continent - into the frozen heart of Antarctica - we find great surprises. This is one of the most volcanic regions on earth, and one of the driest. We reveal unexpected sand dunes, hidden in a rare ice-free valley. Then, on the exposed mountain tops, sticking out from the otherwise ice-covered interior, we find tiny snow petrels, which raise their chicks further south than any other bird, and defend their territory by projectile vomiting!
The greatest revelation lies deep in the interior, beneath the surface of an ice-covered lake, where we discover ancient alien-like structures - giant stromatolites - built by primitive lifeforms. If life can make it here, in the extremes of Antarctica, it raises the possibility that life can exist elsewhere, including in the frozen lakes of distant planets.

When animals can tap into remarkable senses and supernatural skills, crossing the globe, even moving between worlds to find a mate, safety and food, they become truly unstoppable.

As summer spreads across our planet, honey bees toil, snow geese breed, tadpoles awaken and lions stalk wildebeest in search of lush grass.
Our planet is solar-powered. But due to the Earth's tilt, it doesn't strike its surface evenly. This solar energy arrives in varying amounts at different times of the year. The huge increase that comes with summer also triggers countless migrations.

Some animals give their young the best start in life by making truly astonishing journeys. And it's not only where they go that's important but when. Left to fend for themselves until they find their footing, baby sea turtles, elephant seal pups, pumas and crabs bravely trek towards adolescence.