The scientists discovered what they described as a natural plumbing system on the glacier by which meltwater penetrates deeply in the kilometer (0.62 mile) thick ice mass, wrote glaciologists Sarah Das of Woods Hole Oceanographic Institution and Ian Joughin of University of Washington at Seattle.

Thousands of lakes form on top of the Greenland ice sheet each summer. Satellite pictures show that the lakes can vanish in as little as a day, but glaciologists did not know where the water was going or what impact it had on the ice flow.

"We found clear evidence that supraglacial lakes -- the pools of meltwater that form on the surface in summer -- can actually drive a crack through the ice sheet in a process called hydrofracture," wrote Das.

"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."

The lubricating effect of the meltwater can accelerate ice flow 50 to 100 percent in some of the slow-moving areas of the ice sheet, Das and Joughin wrote.

"It's hard to envision how a trickle or a pool of meltwater from the surface could cut through thick, cold ice all the way to the bed," wrote Das.

"For that reason, there has been a debate in the scientific community as to whether such processes could exist, even though some theoretical work has hypothesized this for decades."

In July 2006 instruments that the scientists placed to measure the meltwater movement "captured the sudden, complete draining of a lake that had once covered 5.6 square kilometers (2.2 square miles) of the surface and held 0.044 cubic kilometers (11.6 billion gallons) of water," according to the Woods Hole Oceanographic Institution.

"Like a draining bathtub, the entire lake emptied from the bottom in 24 hours, with the majority of the water flowing out in a 90-minute span. The maximum drainage rate was faster than the average flow rate over Niagara Falls," the scientists wrote.

The research is compiled in two complementary papers and published Thursday in the online journal Science Express, and will appear in the May 9 edition of the magazine Science.

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