On Greenland's ice sheet, a vast icy landscape crisscrossed by turquoise rivers and dotted with meltwater lakes, a small cluster of orange camping tents popped up in late July. The camp, home for a week to a team of researchers, sat by a large, fast-flowing river. Just half a mile (a kilometer) downstream, the river dropped into a seemingly bottomless moulin, or sinkhole in the ice. The low rumble of the waters, the shouted instructions from scientists taking measurements, and the chop of the blades of a helicopter delivering personnel and gear were all that was heard in the frozen landscape....