Planet Earth teems with life. However, not all of the world is particularly friendly to its inhabitants for this is also a planet of extremes: blistering deserts, barren mountain peaks, bleak icy wastelands and dark places that rarely see the sun. In the freezing world of the Arctic, the polar bear is equipped with a coat of fur and blubber designed to cope in a land where winters seem interminable. High altitude mountain ranges also present problems for animals. Mountain goats need to cope with the thin air and treacherous terrain found above the snow line. Life in the desert is a constant battle against a lack of water. The saguaro cactus can expand sufficiently to absorb a ton of water a day when the rains come. In the deepest, darkest caves, freetail bats have developed extraordinarily sensitive hearing. Finally, in the mysterious deep of our oceans, fragile jellyfish are gelatinous enough to thrive in water pressure that would crush most creatures.