An act of Congress established Yellowstone National Park on March 1, 1872. Yellowstone was designated a United Nations international biosphere reserve in 1976 and a world heritage site in 1978.
The park covers more than 2.2 million acres - larger than Delaware and Rhode Island combined. About 96 percent of the park is in Wyoming, but its northern and western borders extend a few miles into Montana and Idaho.
The core of Yellowstone is an ancient volcano crater, at the middle of which lies Yellowstone Lake. The volcano is still somewhat active and about 2,000 small earthquakes rattle the park each year.
The volcanic activity produces Yellowstone's famous geysers, hot springs and various other thermal features. About 10,000 thermal features can be found in the park.