Despite growing evidence of geothermic activity under America's first and foremost national park, it took geologists a long time to realize that there was actually a volcano beneath Yellowstone. And then, why couldn't they find the caldera or crater? Because, as an aerial photograph finally revealed, the caldera is 45 miles wide, encompassing all of Yellowstone. What will happen, in human terms, when it erupts? Greg Breining explores the shocking answer to this question and others in a scientific yet accessible look at the enormous natural disaster brewing beneath the surface of the United States. Yellowstone is one of the world's five "super volcanoes." When it erupts, much of the nation will be hit hard. Though historically Yellowstone has erupted about every 600,000 years, it has not done so for 630,000, meaning it is 30,000 years overdue. Starting with a scenario of what will happen when Yellowstone blows, this fascinating study describes how volcanoes function and includes a timeline of famous volcanic eruptions throughout history.