Strokkur Geyser erupting during a snow storm, Iceland. A geyser is a deep natural well in a geothermal rock fissure. The water lies in a large chamber at the well's base, and is heated by surrounding rocks until it is well over its normal boiling point. The water is prevented from boiling by hydrostatic pressure, and then, at a critical temperature, the superheated water spontaneously boils, resulting in the eruption seen here. These eruptions repeat periodically. Photographed in Geysir, Iceland, the area whose name is the origin of the word geyser.