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.

In a study published today in the online Science Express and scheduled for the May 9 print version of Science, scientists describe using ground-based seismic instruments and water-level monitors, as well as satellites, GPS sensors, and helicopter and airplane overflights to monitor two lakes during the summers of 2006 and 2007. In July 2006, a lake that had covered 2.2 square miles and held 11.6 billion gallons of water suddenly drained like a bathtub: it disappeared in about 90 minutes at a rate of almost four Olympic pools—per second. That’s faster than the average flow rate over Niagara Falls.

The meltwater penetrated the thick Greenland ice, find glaciologist Sarah Das of the Woods Hole Oceanographic Institution and Ian Joughin of the University of Washington. The lake was able to “actually drive a crack through the ice sheet in a process called hydrofracture,” said Das in a statement. “If there is a crack or defect in the surface that is large enough, and a sufficient reservoir of water to keep that crack filled, it can create a conduit all the way down to the bed of the ice sheet.”

“Conduit” is a word you do not want to see anywhere near the phrase “Greenland ice sheet.” There is growing concern that scientists have underestimated the chances that ice sheets and glaciers will not flow out to sea sedately, but will surge, making your nice waterfront home reachable only by snorkeling.

It’s too soon to panic. Yes, the meltwater does lubricate the base on which glaciers and the ice sheet itself rest. “If the ice sheet is frozen to the bedrock or has very little water available, then it will flow much more slowly than if it has a lubricating and pressurized layer of water underneath to reduce friction,” said Das. But although the pressure of the draining lake split the ice sheet from top to bottom, through 3,200 feet of ice, and delivered meltwater directly to the base, the horizontal speed of the ice sheet “only” doubled. “Meltwater does indeed cause substantial speedup” of the ice sheet inland, Joughin told Science, “but it has a small effect on outlet glaciers.”

Some scientists have feared an even greater acceleration. So doubling might not be catastrophic, only (as the scientists write) “substantive.” Or as glaciologist Richard Alley of Pennsylvania State University told Science, “Is it, ‘Run for the hills, the ice sheet is falling in the ocean’? No. It matters, but it’s not huge.”

There is still no good explanation of why Greenland’s ice rivers have accelerated their flow to the sea lately. Das and her colleagues think many other meltwater lakes must be draining and causing the acceleration. Or so they think. Ice sheets have developed an alarming habit lately of surprising scientists. No one knew that a huge meltwater lake could vanish as fast as this one did, or how that will affect the stability of the ice sheet.