(Producer) Episode 4: 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 troglodtes; weird creatures, like the Texas cave salamander; that never see daylight and never set foot on the surface ; Episode 5: Deserts, which make up a staggering 30 percent of the land's surface, seem empty and lifeless, ye they are the most varied of our planet's ecosystems. 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 ; Episode 5: 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, the polar bear is forced to swim vast distances and take on one of the deadliest adversaries on the planet...