The Kyetrak Glacier, located on the northern slope of 26,906-foot Cho Oyu in Tibet, as photographed in 1921 (top) by Major E.O. Wheeler and in 2009 (bottom) by David Breashears. In the past 90 years, the glacier has retreated and melted so extensively that a lake has formed where once there was ice and snow. (Wheeler photo courtesy of Royal Geographical Society.)

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