7. Olympic National Park Location: Washington Size: 922,000 acres Established: June 29, 1938 2015 attendance: 3.3 million Other facts: Ice Age isolation led to 23 unique species (15 animals, 8 plants). The park contains three distinct ecosystems: rugged Pacific shore, subalpine forest, and temperate rain forest. The park is a United Nations World Heritage Site. Found within the park is Mount Olympus, which rises 7,980 feet below sea level.

6. Zion National Park Location: Utah Size: 148,733 acres Established: November 19, 1919 2015 attendance: 3.6 million Other facts: The Virgin River has been responsible for much of the landscape-shaping here. Zion Canyon stretches 15 miles through the park. Horse Ranch Mountain, at 8,726 feet, is the highest point in the park; the lowest point is 3,666 feet, at Coalpits Wash. Kolob Arch, at 284 feet long, is one of the world’s largest freestanding natural arches. More than 100 miles of wilderness trails dot the landscape.

5. Yellowstone National Park Location: Idaho, Montana, Wyoming Size: 2.2 million acres Established: March 1, 1872 2015 attendance: 4.1 million Other facts: This was the first national park in the world. Old Faithful, an uncharacteristically regularly spouting geyser, is one of the world’s most famous attractions. The park is one of the world’s largest wildlife reserves. More geysers and hot springs can be found here than anywhere else on the planet.