Yellowstone National Park’s caldera, which covers a 25- by 37-mile (40- by 60-kilometer) swath of Wyoming, is an ancient crater formed after the last big blast, some 640,000 years ago. The simmering volcano has produced major eruptions—each a thousand times more powerful than Mount St. Helens’s 1980 eruption—three times in the past 2.1 million years. The supervolcano has recently caused miles of ground to rise dramatically, scientists report beginning in 2004, which saw the...







