Outside Antarctica, the world’s greatest storehouse of ice and potential contributor to rising sea levels is Greenland. And its ice sheets are shrinking, both as the edges collapse into the sea and as the two-mile-high surface melts in summer heat—which researchers estimate is the main driver of ice loss. In 2017, two glaciologists used bright dye to track flows of surface water as it plunges into crevasses and naturally formed drain holes called moulins. In places, this water appears to lubricate the interface between ice and bedrock far below and to increase the rate at which ice slides toward the sea. Near Ilulissat, Greenland.