Photographs of Upper Geyser Basin, Yellowstone National Park

Marathon Pool is a neat, circular spring just 70 feet from the Firehole River, measuring ten feet across and containing clear, pale greenish blue water, warm rather than hot, edged by orange-red cyanobacteria. The water level is generally a few inches below the rim, but on very rare occasions it rises and the pool erupts, to a height of six feet. At other times the level falls further, behavior thought to be connected to major eruptions of Grotto Geyser, 300 feet south.

Back to map of the Upper Geyser Basin

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