A serac (originally from Swiss Frenchsérac) is a block or column of glacial ice, often formed by intersecting crevasses on a glacier. Commonly house-sized or larger, they are dangerous to mountaineers since they may topple with little warning. Even when stabilized by persistent cold weather, they can be an impediment to glacier travel.
Seracs are found within an icefall, often in large numbers, or on ice faces on the lower edge of a hanging glacier. Examples of the latter in the Alps are found on the northeast face of Piz Roseg, the north face of the Dent d'Hérens, and the north face of Lyskamm. They are well-known obstacles on many of the world's highest mountains, Kanchenjunga (located on the border of India and Nepal) in particular. The collapse of large seracs was responsible for at least 8 of the 11 deaths of the August 2008 climbing accident on K2. In April 2014, a large serac broke off and caused an avalanche on Mount Everest, which was responsible for the death of 16 climbers.