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Snow petrels are smaller than pigeons but they climb to altitudes of 3,000 metres right onto the vast Antarctic ice cap. The Antarctic ice cap is larger than Australia and the ice is several miles thick. Only the summits of the tallest mountains project through the ice. These are called nunataks. These few patches of rock are isolated in a sea of rock and as precious as an oasis in a desert. Only two percent of the continent is ice free, and nearly all of that is near the coast. Snow petrels cannot lay their eggs on ice and are prepared to fly a very long way to lay their eggs on bare rock. One of their nests was found on a nunatak 140 miles from the coast. They breed farther south than any other bird and have to wait for their nesting ledges to be cleared from the thick snow. Even at the height of summer, the temperature here does not rise above minus 30 degrees. There is no unfrozen water so the birds must bathe in snow to keep themselves clean. As soon as the winds have swept the bulk of the snow from the higher rock slopes, the birds take possession of them. But even then they may have to excavate a metre of snow to get into a crevice and find a nest site that suits them. In the coming season they will have to make the journey to open water again and again to collect food for their chicks. But with their arrival, spring has come to the deep south.