At the top of the 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.

Mountains create frozen habitats on every continent on Earth, and each of these high-altitude worlds holds unique challenges for its surprising and remarkable life. The journey begins close to the equator, the furthest point from the poles, in East Africa. Here on the high slopes of Mount Kenya, during the day the tropical sun keeps the cold at bay, but at night the frost descends.

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. The journey begins at the far edge of the continent, on its far-flung sub-Antarctic islands. Here there are king penguins that, to feed at sea, must face the danger of ferocious leopard seals lurking in the shallows.

In the far north of the planet lies the largest land habitat on earth, home to snow-covered forests and the icy open tundra. These are lands of extremes that push animals to their limits: in winter they are so cold that much of the ground has remained frozen since the last ice age. To stand any chance of survival, animals must adapt in extreme ways: here a super pack of wolves, 25 strong, has come together to take on the only large prey available to them in winter, American bison.

The frozen planet is changing. Meet the scientists and people dedicating their lives to understanding what these changes mean, not just for the animals and people who live there, but for the world as a whole.