Yellowstone National Park, Wyoming, Montana, Idaho

One of Yellowstone National Park's most fascinating features are its geysers. Geysers occur when there is a strong steam vent in an area of actively flowing groundwater. Water flows into the vent blocking the release of steam. The blockage causes underground pressure to build up until it is sufficient to violently eject the water from the vent. The water and built-up steam being ejected is the actual geyser.

Each geyser is unique in its time interval between and during eruptions and how it acts during an eruption. The performance can change from one eruption to the next.

Above left, is the peak eruption of the fittingly named Giant Geyser. During its eruption which lasted about 20 minutes, the geyser actually had many different looks. Top, right: visitors watch a geyser erupt from conveniently placed boardwalks.

Check out the Geyser Slide Show - there are ten photos taken at intervals during this eruption of Giant Geyser.

At left, Yellowstone National Park's most famous geyser is Old Faithful. It is so named because of its regularity, having erupted about once an hour for centuries. (The time interval is actually between about 30 and 90 minutes.) While not the biggest or most spectacular geyser, Old Faithful erupts on a fairly regular schedule and is readily accessible, making it a visitor favorite.

Shown right is the unusual Steady Geyser. Unlike most other geysers which erupt at intervals, this small geyser erupts steadily (hence the name) without stopping.