Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. Yellowstone National Park doesn’t just sit on a volcano—it is a volcano. Underneath the park, red-hot magma reservoirs flow, superheating hot springs and geysers like Old Faithful. This vast volcanic system is known as the Yellowstone Caldera, and with one blast it could plunge the world into chaos.  About two million years ago , as sabertooth tigers and mastodons roamed the future United States, Yellowstone was a powder keg. Large amounts of hot magma accumulated beneath the Earth’s crust, building pressure and volcanic gases that triggered a major eruption.…