The hot springs, mud pots, fumaroles and geysers found at Yellowstone National Park bubble, pop and hiss for a really good reason. Yellowstone sits atop one of the largest volcanoes in the world! Yellowstone Caldera measures 55 km (34 miles) wide by 72 km (45 miles) long and caps a massive chamber of molten rock called magma. And while Yellowstone Volcano hasn't erupted for thousands of years, three eruptions have been supervolcano in size. An eruption 640,000 years ago produced 6,000 times the ash as Mt. St. Helens. The three supervolcanic explosions blew enough ash and lava to fill the Grand Canyon in Arizona! Will Yellowstone Caldera erupt in the future? Scientists think so. But they don't know when. And they don't know if the next eruption will be a supervolcano blast.