Explosions that occur in active volcanoes are called phreatic explosions. This occurs when underground water is suddenly heated by magma or gas. But similar steam explosions, called hydrothermal explosions, can occur in regions that do not have active volcanoes. Like Ontake and White Island, the destructive power comes from water expanding into steam. Yellowstone National Park, where there hasn't been a magma eruption for 70,000 years, has seen hundreds of hydrothermal explosions of various sizes.