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Cryoconite hole bubbles. Close-up of air bubbles in a cryoconite hole on a glacier. A cryoconite hole forms when dust is deposited in ice or snow. The deposit is darker and therefore absorbs solar radiation better than the surrounding area. This melts the snow or ice beneath the deposit, forming a hole. Overnight the water in this hole has frozen over and the thin layer of ice has trapped air bubbles rising from the bottom. Photographed on the Forno glacier, Switzerland (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JUERG ALEAN/SCIENCE PHOTO LIBRARY)