Whooooooshshshshshsh. That sound you hear is Greenland lakes melting through the ice sheet they sit on and greasing the skids for ice to flow out to sea.

That, as everyone knows, raises sea levels. What no one knew is that a lake composed of meltwater and sitting on the Greenland ice sheet can drain as suddenly and completely as scientists have now documented. It adds an ominous twist to a key unknown about glaciers and ice sheets: as the world warms due to the greenhouse effect, will they flow out to sea slowly and gradually, or suddenly and catastrophically?

Every summer, thousands of lakes form atop Greenland’s ice, as sunlight melts the surface. Satellite observations have shown some of the lakes vanishing in a single day, but no one knew where the water went or how it affected the ice sheet’s plumbing— the network of channels and fractures through which meltwater travels.