Segments 4 through 6 of the extraordinary BBC 11-part nature series. 40 camera teams were shooting at over 200 different locations all over the world for more than 5 years to get the pictures seen in the series. Each segment is followed by a sequence investigating the techniques and challenges of wildlife photography within that segment.

Caves: Deep in an undiscovered world, including both poisonous and flooded caves, we find astonishing crystals; cave angel fish, the five million bats of Deer Cave in Borneo; and troglodytes, and weird creatures like the Texas cave salamander, that never sees daylight or sets foot on the surface.

Deserts: 30% of Earth's land surface is desert which can seem empty and lifeless, yet they are the most varied of our planet's ecosystems. With remarkable footage this documentary unravels the secrets behind desert survival and for the first time on such a scale, reveals the ephemeral nature of this dynamic environment.

Ice worlds: The advance and retreat of polar ice is the real challenge to life. As the sea freezes in Antarctica all animals flee, except for the Emperor Penguin. Meanwhile, in the Arctic, humpback whales are feeding and the polar bear swims vast distances, stalking a colony of walruses.