Yellowstone National Park's most famous feature is probably the Old Faithful geyser. It is so named because of its regularity, having erupted about once an hour for centuries. 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. 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.
Pressure builds up underground until it is sufficient to violently eject the water from the vent.
The water being ejected is the actual geyser. Please check out our Yellowstone National Park, II page which features a closer look at geysers. Another exciting feature of Yellowstone National Park is the presence of spectacular waterfalls. The most well known falls is the Lower Falls of the Yellowstone River shown at the top of the page. It flows into the Grand Canyon of the Yellowstone, a very colorful 1,000 foot gorge which runs for several miles.