If the entire Greenland ice sheet, which covers 656,000 square miles, were to melt, it would pour enough water into the world's oceans to raise global sea levels by about 20 feet. But little is understood about how this massive hulk of ice behaves. A group of researchers spent five years measuring and imaging the flow of meltwater across the surface of the ice sheet. They found that each river of meltwater ended in a moulin, a kind of sink hole that allows the water to penetrate into the glacier and drain out at the bottom.

The surface of the ice sheet, in other words, acts a bit like Swiss cheese. ?"One of the most pressing environmental problems of the decades and centuries ahead is rising global sea levels, and one of the leading causes of sea level rise is the melting of ice from glaciers and ice sheets," Laurence Smith, chair of the UCLA geography department and lead author on the research, says. "Greenland in particular is the single largest melting chunk of ice in the world."

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