<P> Yellowstone National Park is a national park located in the U.S. states of Wyoming, Montana, and Idaho . It was established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872 . Yellowstone was the first national park in the U.S. and is also widely held to be the first national park in the world . The park is known for its wildlife and its many geothermal features, especially Old Faithful geyser, one of its most popular features . It has many types of ecosystems, but the subalpine forest is the most abundant . It is part of the South Central Rockies forests ecoregion . </P> <P> Native Americans have lived in the Yellowstone region for at least 11,000 years . Aside from visits by mountain men during the early - to - mid-19th century, organized exploration did not begin until the late 1860s . Management and control of the park originally fell under the jurisdiction of the Secretary of the Interior, the first being Columbus Delano . However, the U.S. Army was subsequently commissioned to oversee management of Yellowstone for a 30 - year period between 1886 and 1916 . 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 a thousand archaeological sites . </P> <P> Yellowstone National Park spans an area of 3,468.4 square miles (8,983 km), comprising lakes, canyons, rivers and mountain ranges . Yellowstone Lake is one of the largest high - elevation lakes in North America and is centered over the Yellowstone Caldera, the largest supervolcano on the continent . The caldera is considered an active volcano . It has erupted with tremendous force several times in the last two million years . Half of the world's geothermal features are in Yellowstone, fueled by this ongoing volcanism . Lava flows and rocks from volcanic eruptions cover most of the land area of Yellowstone . The park is the centerpiece of the Greater Yellowstone Ecosystem, the largest remaining nearly - intact ecosystem in the Earth's northern temperate zone . In 1978, Yellowstone was named a UNESCO World Heritage Site . </P> <P> Hundreds of species of mammals, birds, fish, and reptiles have been documented, including several that are either endangered or threatened . The vast forests and grasslands also include unique species of plants . Yellowstone Park is the largest and most famous megafauna location in the contiguous United States . Grizzly bears, wolves, and free - ranging herds of bison and elk live in this park . The Yellowstone Park bison herd is the oldest and largest public bison herd in the United States . Forest fires occur in the park each year; in the large forest fires of 1988, nearly one third of the park was burnt . 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 snowmobiles . </P>

What formed the central portion of yellowstone national park about two million years ago