A moulin is a sinkhole forming a vertical shaft in the surface of a glacier. It is worn into the ice and sometimes into the rock beneath, by the circular movement of meltwater, swirling down it. The name, from the French word for mill, as in Moulin Rouge, comes from the motion of the water. The following observation by an experienced mountaineer may help one keep in mind the distinction between a crevice and a moulin: “Falls into crevices are often not serious, or at least are survivable (even unroped if you are lucky), but a fall into a moulin is instant death.”

Robert Hass