The plateau developed through three volcanic cycles spanning two million years that included some of the world's largest known eruptions. Eruption of the >2,500 km2 (970 sq mi) Huckleberry Ridge Tuff about 2 million years ago created the more than 75 km (47 mi) long Island Park Caldera. The second cycle concluded with the eruption of the Mesa Falls Tuff around 1.3 million years ago, forming the 25 km (16 mi) wide Henry's Fork Caldera at the western end of the first caldera. Activity subsequently shifted to the present Yellowstone Plateau and culminated 630,000 years ago with the eruption of the 1,000 km2 (390 sq mi) Lava Creek Tuff and the formation of the present 45 km × 75 km (28 mi × 47 mi) caldera. Resurgent doming then occurred at both the NE and SW sides of the caldera and voluminous 1,000 km2 (390 sq mi) intracaldera rhyolitic lava flows were erupted between 150,000 and 70,000 years ago. Phreatic eruptions produced local tephras during the early Holocene. The caldera presently contains one of the world's largest hydrothermal systems including the world's largest concentration of geysers. Much of the plateau is located within Yellowstone National Park.