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.

Researchers have fanned out across Greenland to investigate details of how the lakes might affect the ice sheet and sea level in the future. From recent field studies, they know that when the lakes drain suddenly, they can send meltwater down to the bedrock, where it temporarily lubricates the ice sheet's seaward migration, says Sarah Das, a geophysicist at the Woods Hole Oceanographic Institution. Scientists fear that if the region continues to warm, sudden draining may occur more frequently and over a much broader area of the ice sheet. That could speed the calving of glaciers and contribute to sea-level rise.

Lakes atop the ice sheet also contribute to melting: the ice underneath them thins twice as fast as exposed ice nearby, says Marco Tedesco, a glaciologist at the City College of New York. This summer Tedesco is using a remote-controlled boat to take measurements that will reveal whether the darkness of the lakes correlates with their depth—data that may help analysts better estimate the depths of surface lakes that show up on satellite images and therefore better predict the ice sheet's rate of melting. Laurence C. Smith, a geographer at the University of California, Los Angeles, is comparing rates of surface melting with flow rates in rivers fed by the meltwater. If the two rates diverge substantially, the difference could indicate that some of the meltwater is accumulating below the ice sheet, where it can help to speed the flow of ice to the sea.