Elevation: 7,323 ft.
Giant Geyser is a cone-type geyser in the Upper Geyser Basin of Yellowstone National Park in the United States. Giant Geyser is the namesake for the Giant Group of geysers, which includes Bijou Geyser, Giant Geyser, and Mastiff Geyser. Giant Geyser is also the namesake for the Giant Platform, a raised stone structure incorporating all these geysers. Giant is notable for its spectacular, but sporadic eruptions, as well as for its very large cone of geyserite, which stands about 12 feet tall.
Giant can go long periods between eruptions. Its activity level varies considerably from year to year. For several years starting in 1955 it was dormant, and from 1963 through 1987 only six eruptions were known to have occurred. By contrast, around 1997 the interval between eruptions averaged as little as four days.
The most recent active phase of Giant Geyser started on August 6, 2005 and continued until April 29, 2008, when activity decreased dramatically; there was a single eruption again on August 26, 2008. There were 11 eruptions in 2005, 47 in 2006, 54 in 2007 (the most eruptions for any year since 1955), and 13 in 2008. There were no recorded eruptions in 2009 and 2 in January 2010. The reasons for this variability are unknown.