Yellowstone National Park is a U.S. National Park located in the western states of Wyoming, Montana, and Idaho. Yellowstone is the first and oldest national park in the world and covers 3,845 square miles (8879 km˛), mostly in the northwest corner of Wyoming. The park is famous for its various geysers, hot springs, supervolcano and other geothermal features and is home to grizzly bears, wolves, and free-ranging herds of bison and elk. It is the core of the Greater Yellowstone Ecosystem, one of the largest intact temperate zone ecosystems remaining on the planet. The world's most famous geyser, the Old Faithful Geyser, is also located in Yellowstone National Park.

Long before any recorded human history in Yellowstone, a massive volcanic eruption spewed an immense volume of ash that covered all of the Western U.S., much of the Midwestern U.S., Northern Mexico and some areas of the Pacific Coast. The eruption dwarfed that of Mount St. Helens in 1980 and left a huge caldera 43 miles by 18 miles (70 km by 30 km) sitting over a huge magma chamber (see Geology section and Yellowstone Caldera). Yellowstone has registered three major volcanic eruption events in the last 2.2 million years with the last event occurring 640,000 years ago. Its eruptions are the largest known to have occurred on Earth within that timeframe, producing drastic climate change.