Old Faithful is the most frequently erupting large geyser in Yellowstone National Park. A geyser is a hot spring which erupts periodically. These eruptions are caused by the buildup of hot water and steam trapped by constrictions in the "plumbing system" of a hot spring. When enough pressure builds up the geyser erupts. See the National Park Service site "Geysers and How They Work" to learn more.
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