Yellowstone National Park (Arapaho: Henihco'oo' or Héetíhco'oo) is a national park located primarily in the U.S. state of Wyoming, although it also extends into 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, widely held to be the first national park in the world, is known for its wildlife and its many geothermal features, especially Old Faithful Geyser, one of the most popular features in the park. It has many types of ecosystems, but the subalpine forest is dominant.
Native Americans have lived in the Yellowstone region for at least 11,000 years. The region was bypassed during the Lewis and Clark Expedition in the early 19th century. Aside from visits by mountain men during the early-to-mid-19th century, 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.