The most famous feature at Yellowstone, Old Faithful spouts every 74 to 79 minutes. It's an impressive sight, even though you will generally be viewing it with a crowd. Get up in the middle of the night or very early in the morning to experience it on your own.
Old Faithful is in what's called Upper Geyser Basin. There are a number of other geysers close by that may be less reliable than Old Faithful but can be equally impressive. Castle Geyser is named for its shape—a large cone. North of Old Faithful, the Lower Geyser Basin contains other geothermal features, including fumaroles and hot springs. Fountain Paint Pot is the most popular of several mud pots in the area—it "decorates" the basin in spurts of earth tones. On the Grand Loop, Yellowstone National Park. http://www.nps.gov/archive/yell/tours/oldfaithful/index.htm.
