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Geyser in Geysir, Iceland. Photo: Julien Carnot


A natural, hot-water underground spring that frequently erupts with a stream of hot water and steam. Most geysers erupt irregularly. Other geysers, such as Old Faithful in Yellowstone National Park, erupt at fairly regular intervals. [Geyser was named for a hot spring in Iceland and means “gusher” in Icelandic.]