Wyoming, Montana and Idaho
Visitors in 2009 (through August): 2.7 million
Established in 1872, Yellowstone is the nation's oldest national park. It's known for its geysers and hot springs. Indeed, Yellowstone contains 60% of the world's geysers, including Old Faithful, its most famous, and the Grand Prismatic Hot Spring, America's largest hot spring. The park also houses a rich collection of historical artifacts in its museum, library and research centers.
Pictured: Morning Glory Pool Hot Spring.