One of the most fascinating occurrences in nature is the sudden eruption of steam and a spewing column of boiling water known as a geyser. Geysers are extremely rare and require a complex set of circumstances to occur. In the simplest of terms, there essential conditions include a water supply, a heat source and a system of underground shafts or "standpipes" through which the steam erupts. Some geysers, such as Old Faithful in Yellowstone National Park, Wyoming, erupt at regular, predictable intervals. There are only about 50 known geyser fields in existence; Yellowstone has more geysers then any other fields in the world.