Yellowstone lies inside the caldera of one of the largest volcanoes in the world, which has erupted at least three times, and volcanic activity remains active below ground. Magma moving underground is pushing up the hill in front of the viewer, and two of these bulges, called resurgent domes, have been found in the caldera; part of the Sour Creek dome, an oval-shaped hill about ten miles long and six miles wide, can be seen from here. Within Yellowstone's caldera are thousands of mudpots, fumaroles, hot springs, and geysers. The caldera, thirty miles wide and forty-five miles long, is so large that the massive eruption that created it is difficult to imagine, and after the volcano exploded, lava continued to flow, filling much of the caldera and making the crater and its rim hard to see. By comparing new surveys with benchmark elevations, geologists have determined that the Yellowstone caldera is rising and falling as magma and hydrothermal fluids move below.