Only the most powerful icebreakers can force a passage through the vast band of broken ice that rings Antarctica. In places it is 200 miles across. Yet it is home to the most numerous large mammal in the world other than man - the crabeater seal. Up to 30 million of them live around the continent between the ice and the water. But they never come to land. Despite their name, they live on krill, sieving seawater through their interlocking teeth to consume 20 kilos of krill every day.