Geysir, geyser located in the Hauka valley (Haukadalur), southwestern Iceland. The spouting hot spring gave its name (in use since 1647) to similar phenomena around the world. It spouted boiling water at least as early as the 13th century, but since 1916 it has been relatively inactive because of a buildup of sediment in the underground water passages. Its circular pool is 60 feet (18 m) in diameter and 4 feet (1.2 m) deep. Although it now spouts much less often than some other geysers in the vicinity, its height occasionally reaches nearly 200 feet (61 m).