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A serac (originally from Swiss French sérac) is a block or column of ice formed by intersecting crevasses on a glacier. Often 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 a well-known obstacle on many of the world's highest mountains, Kanchenjunga in particular. The collapse of large seracs was responsible for at least 8 of the 11 deaths of the August 2008 K2 climbing accident on the world's second highest mountain, K2.