Yellowstone National Park is home to some 10,000 thermal features, over 500 hundred of which are geysers. In fact, Yellowstone contains the majority of the world’s geysers. Geysers are hot springs that erupts periodically. The eruption is the result of super-heated water belowground becoming trapped in channels leading to the surface. The most famous geyser is probably The Oldfaithful.
In this photo tourists are walking in front of one such erupting geyser.