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Old Faithful geyser. Eruption of steam and hot water from Old Faithful geyser. Geysers consist of deep natural wells in geothermal rock fissures. Their water lies in a large chamber at the base of the well. The water is heated by the surrounding rocks until it is well over its normal boiling point. However, it is prevented from boiling by hydrostatic pressure. At a critical temperature, the superheated water spontaneously boils, causing an eruption. Old Faithful is in Yellowstone National Park, Wyoming, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER)