Geyser, hot spring, found in currently or recently volcanic regions, that intermittently jets steam and superheated water into the air. It consists essentially of a system of underground tubelike fractures leading down to a heat source. Groundwater accumulates in the tube, that near the bottom being kept from boiling by the pressure of the cooler layers above. When the critical temperature is reached, bubbles rise, heating the upper layers, which expand and well out of the orifice. This reduces the pressure enough for substantial steam formation below, with subsequent eruption. The process then recommences. The famous Old Faithful in Yellowstone National Park used to erupt every 66.5 min, but has recently become less reliable.