A crevasse is a deep crack or split in a glacier, the massive rivers of ice that flow slowly down mountains in places like Alaska, Antarctica, or the Alps. Crevasses form when the glacier moves over uneven ground or bends around curves, causing the ice to stretch and crack apart. Some crevasses are only a few feet deep, but others plunge hundreds of feet down into blue-green darkness.

Mountain climbers and polar explorers treat crevasses with serious respect because they're dangerous. Fresh snow can hide a crevasse's opening, creating a thin bridge that looks solid but might collapse under a person's weight. That's why experienced climbers rope themselves together when crossing glaciers: if one person falls into a crevasse, the others can help pull them out.

Don't confuse it with crevice, which is a narrow crack in rock or a wall. A crevice might be somewhere a lizard hides; a crevasse is large enough to swallow a person, a tent, or even a vehicle. When you read about Antarctic expeditions or mountaineering adventures, crevasses are often one of the most frightening obstacles explorers face.