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ike holes that form in glaciers, funnel meltwater from the ice surface to the ground beneath, and they are the alarmingly efficient conduits that allow surface water to reach deep and drive the ice to flow faster. “Forming a moulin in Greenland requires a crack on the ice surface that becomes filled with water that eventually drives the crack through the ice. But there’s a mystery here: A large fraction of the moulins in Greenland form some distance away from the ice sheet's existing crevasse fields,” said Matthew Hoffman, a Los Alamos glaciologist and corresponding author on a paper this week in Geophysical Research Letters (LINK), coauthored by researchers from Sandia National Laboratories, NASA Goddard Space Flight Center, and the universities of Montana, Texas and Zurich. The word “moulin” is French for “mill,” so-called because the water cascading into a glacial moulin resembles water cascading over a water wheel at a grain mill. “To explore the moulin formation question, our team combined on-site measurements from GPS stations with computer simulations driven by the hourly GPS data,” said Hoffman. “We determined that the key is in the meltwater lakes that can form on the glacier surface, and that during the summer, occasionally have catastrophic drainage events, in which they empty their contents down through the ice in a matter of hours.”