Yellowstone National Park

The vast natural forest of Yellowstone National Park covers nearly 3,475 square miles ; 96% of the park lies in Wyoming, 3% in Montana and 1% in Idaho. Yellowstone contains half of all the world's known geothermal features, with more than 10,000 examples. It also has the world's largest concentration of geysers (more than 300 geyers, or two thirds of all those on the planet). Established in 1872 as the first National Park in the US, Yellowstone is equally known for its wildlife, such as grizzly bears, wolves, bison and wapitis.
Source: UNESCO World Heritage Convention website