S2, E1: 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. Only in spring, 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. But it is a brief respite, for the ice soon returns.