A vertical shaft in a glacier that carries meltwater downward is called a moulin.
A moulin forms when surface meltwater finds or creates a weakness in the glacier, often at a crevasse, and drills downward through the ice. The shaft is typically circular or nearly circular and may carry water from the surface into the glacier's interior or all the way to its bed.
Moulins can be only tens of metres deep or extend hundreds of metres through an ice sheet. Their French-derived name means “mill,” reflecting the way flowing water can carve a shaft through the ice. The water often continues through englacial and subglacial channels before emerging near the glacier terminus.
Meltwater entering a moulin can affect glacier motion by raising water pressure at the bed and reducing friction, although the effect depends on the drainage system. A randkluft is a gap between glacier ice and rock, a serac is a large ice block, and a kettle is a glacial-depression feature—not a meltwater shaft.