Every summer near-perpetual sunlight pours down on much of the ice-swaddled island of Greenland. On many parts of the ice sheet, especially at lower elevations, meltwater flows across the surface and collects in deep-blue ponds and lakes, such as the one shown here. Unlike the lakes we swim in, these water bodies can disappear in a wink: a lake that would fill the Superdome in New Orleans more than a dozen times can drain through a crack in the ice in just 90 minutes.