Episodes 4-6 of this 11-part series. In Caves, Planet Earth descends deep into an undiscovered, underground world to introduce some of the most remarkable and bizarre animals ever seen as well as astonishingly beautiful caverns and tunnels. In Deserts, with remarkable footage, Planet Earth unravels the secrets behind desert survival and, for the first time on such a scale, reveals the ephemeral nature of this dynamic environment. In Ice worlds, the advance and retreat of polar ice is the real challenge to life in the Arctic and Antarctic. For most of the year, these regions are locked in darkness and ice, but when the sun returns, this cold desolate wasteland blossoms with life.