Iceberg, Jakobshavn Fjord, Greenland, 2007 Photograph by James Balog

At four miles (six kilometers) wide and several thousand feet thick Jakobshavn Isbrж disgorges icebergs like these faster than any other of Greenland's glaciers. Its output, accelerated by global warming, totals some 11 cubic miles (45 cubic kilometers) of ice each year.

(Photo shot on assignment for, but not published in, "The Big Thaw," June 2007, National Geographic magazine)

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