Lassen Volcanic National Park

Location: California

Year Established: 1916

Area: 106,372.36 acres (430.5 km2)

Annual Visitors: 432,977

Lassen Peak, the largest plug dome volcano in the world, is joined by all three other types of volcanoes in this park: shield, cinder dome, and composite. Though Lassen itself last erupted in 1915, most of the rest of the park is continuously active: numerous hydrothermal features, including fumaroles, boiling pools, and bubbling mud pots, are heated by molten rock from beneath the peak.