A moulin is a vertical shaft or deep hole on the surface of a glacier that acts like a drain, funneling meltwater deep into the glacier, sometimes all the way to the bed. They are formed by surface meltwater that finds and erodes a path down through the ice, and the rushing water, often carrying debris, can widen and deepen the shaft over time. Moulins are also known by terms like “glacial mill” or “glacier sinkhole”.
Mountaineering and Snow Climbing, Glacier Travel and Crevasse Rescue
Moulins and Glaciers
American Alpine Institute