Yellowstone National Park, set aside as a national park on March 1, 1872, is located mostly in the U.S. state of Wyoming, though it also extends into Montana and Idaho. The park was the first of its kind, and is known for its wildlife and geothermal features, especially Old Faithful Geyser, one of the most popular areas in the park.
Aside from visits by mountain men during the early to mid-1800s, organized exploration did not begin until the late 1860s. The U.S. Army was commissioned to oversee the park just after its establishment. In 1917, administration of the park was transferred to the National Park Service, which had been created the previous year. Hundreds of structures have been built and are protected for their architectural and historical significance, and researchers have examined more than 1,000 archaeological sites.
Yellowstone National Park spans an area of 3,472 square miles (8,987 km²), comprising lakes, canyons, rivers and mountain ranges. Yellowstone Lake is one of the largest high-altitude lakes in North America and is centered over the Yellowstone Caldera, the largest supervolcano on the continent. Half of the world's geothermal features are in Yellowstone, fueled by this ongoing volcanism. The park is the centerpiece of the Greater Yellowstone Ecosystem, the largest remaining, nearly-intact ecosystem in the Earth's northern temperate zone.
Hundreds of species of mammals, birds, fish and reptiles have been documented. The vast forests and grasslands also include unique species of plants. Grizzlies, wolves, and free-ranging herds of bison and elk live in the park. Forest fires occur in the park each year; in the large forest fires of 1988, nearly one third of the park burned. Yellowstone has numerous recreational opportunities, including hiking, camping, boating, fishing and sightseeing. Paved roads provide close access to the major geothermal areas as well as some of the lakes and waterfalls. During the winter, visitors often access the park by way of guided tours that use either snow coaches or snowmobile.