the oldest, one of the largest, and probably the best-known national park in the United States. It is situated in northwestern Wyoming and partly in southern Montana and eastern Idaho and includes the greatest concentration of geothermal features in the world. The park was established by the U.S. Congress on March 1, 1872, as the nation's—and the world's—first national park. It was designated a World Heritage site in 1978. The park, almost all of which remains undeveloped, covers 3,468 square miles (8,983 square km) and consists mostly of broad volcanic plateaus with an average elevation of 8,000 feet (2,440 metres). Among the mountain ranges that protrude into the park are the Gallatin Range on the northwest, the Absaroka Range on the east, and the Tetons (Teton Range) along the park's southern boundary. The park is also surrounded by the Custer, Shoshone, Teton, Targhee, Beaverhead, and Gallatin national forests.

Aside from its rugged mountains and spectacular deep valleys, Yellowstone has unusual geologic features, including fossil forests, eroded basaltic lava flows, a black obsidian (volcanic-glass) mountain, and odd erosional forms. Its principal attractions, however, are its 10,000 hot springs, which find surface expression as steam vents, fumaroles, colourful hot pools, mud cauldrons, paint pots, hot springs and terraces, hot rivers, and geysers. Of the park's more than 200 geysers, many erupt to heights of 100 feet (30 metres) or more. Old Faithful, the most famous geyser in Yellowstone, erupts fairly regularly every 33 to 120 minutes. Many of Yellowstone's noted hot springs and geysers are located in the western portion of the park between Old Faithful and Mammoth Hot Springs (the park's headquarters) some 50 miles (80 km) north. These include The Giantess, a geyser near Old Faithful with a six- to eight-month wait between eruptions; the Fountain Paint Pots in the Lower Geyser Basin, with pink, plopping mud geysers, fumaroles, and a blue, hot-spring pool; and Minerva, a multicoloured terrace with hot water cascades, at Mammoth Hot Springs.

Yellowstone is also known for its lakes and rivers; among these are Yellowstone Lake, Shoshone Lake, the Snake River, the Lewis River, and the Yellowstone River. Yellowstone Lake has a surface area of 136 square miles (352 square km) and is the largest mountain lake in North America. Fishing and boating are popular. The Grand Canyon of the Yellowstone River is a colourful gorge 19 miles (30 km) long located in the north-central part of the park. The canyon, between 800 and 1,200 feet (240 and 370 metres) deep and up to 4,000 feet (1,200 metres) wide, has brilliantly coloured rock walls, with two majestic waterfalls along its course; the Lower Falls of the Yellowstone River have a drop of 308 feet (94 metres).

The park has more than 500 miles (800 km) of roads and more than 1,000 miles (1,600 km) of trails. The John D. Rockefeller, Jr., Memorial Parkway, an 80-mile (130-km) scenic roadway that was established in 1972, connects Yellowstone with Grand Teton National Park to the south.

Robert B. Keiter and Mark S. Boyce (eds.), The Greater Yellowstone Ecosystem (1991), collects papers on resource management policies. The 1988 fires are the subject of Micah Morrison, Fire in Paradise: The Yellowstone Fires and the Politics of Environmentalism (1993). Michael K. Phillips and Douglas W. Smith, The Wolves of Yellowstone (1996), with over 70 color photos; and Thomas McNamee, The Return of the Wolf to Yellowstone (1997), discuss the reintroduction of wolves to the park. Photographic works include Lee H. Whittlesey et al., A Yellowstone Album: A Photographic Celebration of the First National Park (1997); and Mary Meagher and Douglas B. Houston, Yellowstone and the Biology of Time: Photographs Across a Century (1998), comparing 19th-century photographs with recent shots of the same scenes.