Yellowstone National Park sits atop a giant “supervolcano” known as the Yellowstone Caldera, spanning parts of Wyoming, Idaho and Montana, where the magma is just three to eight miles below the surface. Though rare, past eruptions have been massive, with the largest two occurring millions of years ago. The 30-by-45-mile caldera (defined as a large, cauldron-like hollow formed when a volcano collapses inward) has been undergoing episodes of uplift (rising) and subsidence (sinking) for thousands of years, resulting in ground deformations that significantly alter the Earth’s surface.

Scientists believe this activity is a type of natural pressure release, and the chances of an eruption are extremely low. Nevertheless, they continually monitor the area for seismic activity using radar maps, GPS satellites and 17 ground stations. About 1,500 to 2,000 earthquakes are documented annually, and a 19-mile-wide dome was recently discovered at the northern rim of the Yellowstone Caldera, in northwest Wyoming. While this bulge is expected to reverberate for years, experts believe it is unlikely to erupt.