Life on the edge of a frozen sea is tough. Pack-ice at both poles is constantly on the move, and in winter freezes solid with air temperatures 70°C below freezing. Only in spring, with the retreating ice and light reaching the water, does life begin again. Plankton blooms and feeds vast hordes of migrating fish, birds, whales, seals and polar bears. Walruses rake the seabed for clams. Minke and humpback whales gorge themselves on gigantic swarms of krill. But it is a brief indulgence, for the ice soon returns and pushes life back into the ocean.