Lassen Volcanic National Park

All four types of volcano — shield, composite, cinder cone, and plug dome — exist within this single park. 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 a fraction of nearby Yosemite. Winter buries the roads under 30 feet of snow, making it one of the most seasonal parks in the system.