Details from a panorama of the West Rongbuk Glacier and Mount Everest, from a 1921 photo (left) by Major E.O. Wheeler and from a 2008 photo (right) by David Breashears. The glacier, located at an elevation of 17,300 to 24,400 feet, has experienced an average vertical loss of 340 feet in the past century. (Wheeler photo courtesy of Royal Geographical Society.)

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