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Each spring and summer, as the air warms up and the sunlight beats down on the Greenland ice sheet, sapphire-colored ponds spring up like swimming pools. As snow and ice melt atop the glaciers, the water flows in channels and streams and collects in depressions on the surface that are sometimes visible in satellite imagery. These melt ponds and lakes can disappear quickly – a phenomenon that scientists have observed firsthand in recent years. Photo courtesy of NASA.