Simmering below the geysers and hot springs of Yellowstone National Park in Wyoming is a gigantic reservoir of magma that someday could erupt. To find out how much ash such an explosion would generate, the U.S. Geological Survey gathered data and ran a simulation about the distribution of ash from prior supervolcano eruptions, including those that happened at Yellowstone 2.1 million, 1.3 million and 640,000 years ago. A January eruption lasting one month would bury nearby states in a meter or more of the thin, abrasive debris. It would also cast a blanket of at least a few millimeters over the entire U.S. and much of Canada—enough, according to the usgs, to damage airplanes and cars, shutting down transportation. A supervolcano has erupted somewhere on the planet every 100,000 years or so, but usgs experts say that current geologic activity at Yellowstone “provides no signs that a supereruption will occur in the near future.”