Yellowstone National Park

Yellowstone is a huge park in the United States.

In some places, hot water bubbles up from under the ground. Some pools are bright blue, orange and yellow. Some spouts shoot hot water high into the air. Bison, bears and wolves live in the park too.

Yellowstone National Park lies mostly in the state of Wyoming, with small parts in Montana and Idaho. It is about 102 kilometers from north to south, and it has more than 10,000 hydrothermal features: places where hot water or steam comes out of the ground. More than 500 of them are geysers, hot springs that shoot water into the air.

In 1872 the US Congress set Yellowstone aside as a national park. UNESCO calls it the world's first national park, and in 1978 it became a World Heritage Site. People come to see its bubbling pools, its canyons and waterfalls, and its herds of wild animals.

The National Park Service gives Yellowstone an area of 3,472 square miles (8,991 km²), stretching 63 air miles (102 km) north to south and 54 (87 km) east to west, from 1,610 m at Reese Creek to 3,462 m on Eagle Peak. UNESCO puts 96 percent of it in Wyoming, 3 percent in Montana and 1 percent in Idaho. It was established on March 1, 1872, and UNESCO describes it as designated by Congress as the world's first national park; it is managed under the Organic Act of 1916 that created the Park Service.

The park counts more than 10,000 hydrothermal features and more than 500 active geysers, which the Park Service says is more than half the world's geysers. (UNESCO's short description of the site gives a lower count of more than 300.) Yellowstone Lake, at 2,357 m (7,733 ft), covers 341.1 km² and reaches 131 m deep. UNESCO inscribed the park in 1978 for its scenery, its geology, its intact ecosystem and its refuge for rare species.

Rain soaks down into the hot ground.

The hot rock heats the water. Then the water comes back up as a hot pool. Some pools are rainbow colored. Tiny living things make many of the colors!

Yellowstone's hot water starts as rain. It seeps down through the rock, gets superheated by the magma system, and rises again. In a hot spring, the water flows up freely and keeps circulating, so it never builds up enough pressure to erupt. A geyser has narrow spots in its underground plumbing, so pressure builds until the water bursts out.

Grand Prismatic Spring is Yellowstone's largest hot spring. Its bright rings are made by thermophiles, tiny heat-loving living things grouped together by the trillions. Colorless and yellow kinds grow in the hottest water; orange, brown and green kinds grow where it is cooler.

The Park Service describes five visible types of hydrothermal feature. Hot springs, the most common, have open plumbing: convection keeps superheated water circulating so it never reaches eruption. Geysers have constrictions, so pressure builds and is released in eruptions. Mudpots are acidic springs short of water, where microbes help turn hydrogen sulfide into sulfuric acid that breaks rock into clay. Fumaroles boil dry before reaching the surface and can reach 138 °C. Travertine terraces, as at Mammoth Hot Springs, form where hot water dissolves limestone and deposits calcite.

Grand Prismatic Spring, in Midway Geyser Basin, is the park's largest hot spring; the Park Service gives 200 to 330 feet across and more than 121 feet deep, and the USGS gives 370 feet at its widest, calling it the largest hot spring in the United States. Its center is near boiling and blue. Bands of thermophiles, each adapted to a narrow temperature range, color the edges. Trapper Osborne Russell's 1839 account of a 'boiling lake' is, according to the USGS, the earliest written description of a Yellowstone thermal feature that can be definitely identified.

Big, shaggy bison live in Yellowstone.

They have lived there for a very, very long time. Wolves and bears live there too. Elk, birds and fish live in the park as well.

Yellowstone has 67 kinds of mammals, about 330 kinds of birds and two kinds of bears, grizzly bears and black bears. It is the only place in the United States where bison have lived since prehistoric times. Bison almost died out in the 1800s, and in 2025 the park estimated nearly 5,300 lived there. A big male can weigh up to 900 kilograms.

Wolves were gone from the park by 1926. In January 1995, 14 gray wolves were flown in from western Canada and set free after about ten weeks in pens. UNESCO says bringing wolves back helped many living things, from aspen trees to trout.

The Park Service lists 67 mammal species, including seven native ungulates and two bears, about 330 bird species (150 nesting), 16 fish species (five nonnative) and two threatened species, the Canada lynx and grizzly bear. More than 80 percent of the forest is lodgepole pine. UNESCO describes the park as one of the few remaining intact large ecosystems in the northern temperate zone, where lightning fires are often allowed to burn as part of natural succession.

Yellowstone is the only place in the lower 48 states with a continuously free-ranging bison population since prehistoric times; market hunting and the US Army nearly wiped bison out in the 1800s. The park's 2025 estimate was nearly 5,300, in northern and central breeding herds. Wolves were eliminated by 1926; in January 1995 biologists brought 14 gray wolves from western Canada, and the Park Service says 41 wolves from Canada and northwest Montana were released from 1995 to 1997. UNESCO credits the restoration with positive effects on species and processes from aspen to trout. Smithsonian Magazine reported in 2026 that the park has averaged about 100 wolves in ten packs over the past dozen years, according to senior wildlife biologist Dan Stahler, who also cautions that some early accounts of the wolves' effect on plants went too far: willow and aspen have grown back less than once thought, mainly because there are fewer elk.