A geyser is an intermittent spout of geothermally heated groundwater. The word geyser comes from the name of a single Icelandic geyser, Geysir, written mention of which dates back to A.D. 1294.

Some geysers erupt periodically, others irregularly; a few send jets of water and steam hundreds of feet into the air, others only a few feet. There are fewer than 700 geysers in the world, all concentrated in a few dozen fields. More than 60% of the world's geysers are in Yellowstone National Park in the northwestern United States, including the famous geyser, "Old Faithful."

Geysers form only under special conditions. First, a system of underground channels must exist in the form of a vertical neck or series of chambers. The exact arrangement cannot be observed directly, and probably varies from geyser to geyser. This system of channels must vent at the surface. Second, water deep in the system...