(Photo: Associated Press)
Yellowstone National Park is in the northwestern corner of Wyoming, along with a little bit spilling over into Idaho and Montana. At 3,468 square miles, it's larger than the states of Delaware and Rhode Island together.
(Image: National Park Service)
Yellowstone rests on a geological hot spot--specifically, a volcanic caldera. The enormous heat contained beneath the Earth's surface bubbles up through geysers and hot springs.
The geothermal heat can damage human structures on the surface. This photo from the Associated Press shows a strip of Firehole Lake Drive. The subsurface heat has melted the asphalt.
P.S. Harry Turtledove, a fantasy and alternate history novelist, wrote a trilogy about life in the United States after a massive Yellowstone eruption. The first book is called Supervolcano: Eruption.