As the deep water becomes hotter, and consequently lighter, it suddenly rises to the surface and shoots out of the surface in the form of steam and hot water. In turn, the explosion agitates all the water in the geyser reservoir, creating further explosions. Immediately afterward, the water again flows into the underground reservoir, heating begins, and the process repeats itself.
In order to function, then, a geyser must have a source of heat, a reservoir where water can be stored until the temperature rises to an unstable point, an opening through which the hot water and steam can escape, and underground channels for resupplying water after an eruption.
Favorable conditions for geyser exist in regions of geologically recent volcanic activity, especially in areas of more than average precipitation. For the most part, geysers are located in three regions of the world: New Zealand, Iceland, and the Yellowstone National Park area of the United States. The most famous geyser in the world is Old Faithful in Yellowstone Park. Old Faithful erupts every hour, rising to a height of 125 to 170 feet and expelling more than ten thousand gallons during each eruption. Old Faithful earned its name because, unlike most geysers, it has never failed to erupt on schedule even once in eighty years of observation.