Yellowstone National Park
The basics: The country’s first national park, in Montana, Wyoming and Idaho, shows off volcanic forces with fantastical geysers, hot springs and mud spots. It holds the largest concentration of mammals in the lower 48 states, offering up sightings of black bear, bison, pronghorn sheep, wolves and other species.
Visitors: 4.09 million people in 2015.
Square miles: 3,486.
More information: nps.gov/yell; 1-307-344-7381.