A crevasse is a deep, wedge-shaped opening in a moving mass of ice called a glacier.

Crevasses usually form in the top 50 meters (160 feet) of a glacier, where the ice is brittle. Below that, a glacier is less brittle and can slide over uneven surfaces without cracking. The inflexible upper portion may split as it moves over the changing landscape.

Crevasses also form when different parts of a glacier move at different speeds. When traveling down a valley, for example, a glacier moves faster in the middle. The sides of a glacier are slowed down as they scrape against valley walls. As the sections advance at different speeds, crevasses open in the ice.

A bergschrund is a special type of crevasse. Bergschrunds are cracks that appear between the moving ice of a glacier and the non-moving, or stagnant, ice of a mountain or cliff.

Crevasses may stretch across a glacier, run along its length, or even crisscross it. Some crevasses have measured as large as 20 meters (66 feet) wide and 45 meters (148 feet) deep.

Crevasses, which are usually deep, steep, and thin, are a serious danger for mountaineers. Sometimes, a thin layer of snow may form over a crevasse, creating a snow bridge. Snow bridges blend in with the surrounding landscape, hiding the crevasse. Thin snow bridges usually cannot hold a person's weight, so mountaineers secure themselves to each other with rope. All experienced mountaineers are trained in crevasse rescue.

The Khumbu Icefall, part of a massive glacier on the south slope of Mount Everest in Nepal, is one of the most difficult obstacles for mountaineers. The Khumbu glacier moves very rapidly, and crevasses open quickly. A series of ladders and ropes helps many climbers, but the area is still one of the most dangerous on the mountain.

Crevasses can create seracs, which are also dangerous to mountaineers. Seracs are tall pillars formed where several crevasses once met.

A serac can be as large as a house, and may topple with little warning. Icefalls often have dozens of seracs.