From the World Heritage inscription:
The vast natural forest of Yellowstone National Park covers nearly 9,000 km2; 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, Yellowstone is equally known for its wildlife, such as grizzly bears, wolves, bison and wapitis.
Not only is Yellowstone the first national park in the world, but it is also one of the best. You’d be hard pressed to make a top 10 list of attractions in the natural world and not put Yellowstone on the list. Between the geology and the wildlife, Yellowstone has it all.