A plume of molten rock that rises beneath Yellowstone creates one of the world’s largest active volcanoes, evident in the frequency of earthquakes in the area, thousands of geothermal features, and visible ground deformation over time. The first major eruption of the Yellowstone volcano, which occurred 2.1 million years ago, is among the largest volcanic eruptions known, covering over 15,000 sq. km with ash. After the most recent major eruption, 640,000 year ago, the ground collapsed into the magma reservoir, leaving a caldera 75 km long and 55 km wide. Since then, 80 smaller eruptions have occurred, most recently 70,000 years ago, partially filling the caldera. Although a cataclysmic eruption is considered unlikely in the foreseeable future, continuous monitoring of seismic activity, geothermal features, and ground deformation helps to ensure public safety. The Yellowstone Volcano Observatory was created as a partnership among the U.S. Geological Survey, Yellowstone National Park, and University of Utah to strengthen the long-term monitoring of volcanic and earthquake unrest in the Yellowstone area.