An environment where only the toughest survive, the Arctic and Antarctic are unrelenting habitats. Only in spring does life begin again. Plankton blooms and feeds fast hordes of migrating fish, walruses rake the seabed for clams, and minke and humpback whales gorge themselves on gigantic swarms of krill. But it is a brief spring feast - the ice soon returns and pushes life back into the ocean.