Yellowstone National ParkAmerica's first national park — Old Faithful, Grand Prismatic, and a supervolcano caldera

Yellowstone is the original national park — Congress passed and President Grant signed the founding act on March 1, 1872, fourteen years before the first National Park Service ranger was hired and forty-four years before the agency itself was created. (The U.S. Army administered the park from 1886 until the NPS took over in 1918.) The 2.2-million-acre park sits at the corner of Wyoming, Montana, and Idaho on top of an active supervolcano caldera that has erupted three times in the last 2.1 million years.

Geothermal features — hot springs, geysers, fumaroles, mudpots — number more than 10,000 inside the park, with over 500 active geysers. That's roughly half the world's total. Old Faithful erupts 90 feet into the air every 90 minutes (give or take ten); the Grand Prismatic Spring is the largest hot spring in the U.S. and the third-largest in the world; Castle Geyser, Steamboat (the world's tallest active geyser), and the Mammoth Hot Springs travertine terraces round out the front-country features.

Yellowstone is also America's largest near-intact temperate ecosystem — wolves were reintroduced in 1995 and are now thriving in Lamar Valley (the "Serengeti of North America"); about 4,000 to 5,000 bison roam the largest free-ranging herd in the country, the only place in the U.S. they have lived continuously since prehistoric times; grizzlies, elk, moose, bighorn, pronghorn, and lynx fill out the ecosystem. Five entrance stations feed the 142-mile Grand Loop Road, the figure-eight route that connects every major geothermal basin and canyon overlook.