Lassen Volcanic National Park

All four types of volcano — shield, composite, cinder cone, and plug dome — exist within this single park, a geological variety that outperforms #3 Yellowstone National Park, which contains only three types. Lassen Peak last erupted in 1915, and the hydrothermal areas are still violently active: Bumpass Hell is a boardwalk through boiling mud pots, fumaroles, and acid pools. The park gets about 500,000 visitors — respectable, but still just 8% of nearby Yosemite's annual 3.9 million. Winter buries the roads under 30 feet of snow, making it one of the most seasonal parks in the system and limiting peak access to July through September.