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Glacial cave. Man looking at air pockets in the underside of the Aletsch Glacier, from an ice cave formed beneath it. The Aletsch Glacier is the largest glacier in the Alps, with a length of around 23km and covering more than 120 square kilometres. It lies in the eastern Bernese Alps, Valais, Switzerland. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JUERG ALEAN/SCIENCE PHOTO LIBRARY)