The hot molten rock beneath America's Yellowstone National Park is 2.5 times larger than previously estimated, meaning the park's supervolcano has the potential to erupt with a force about 2,000 times the size of Mount St Helens, according to a new study. By measuring seismic waves from earthquakes, scientists were able to map the magma chamber underneath the Yellowstone caldera as 88 kilometres long, lead author Jamie Farrell of the University of Utah said. The chamber is 29 kilometres wide and runs at depths from five to 14.5 kilometres below the earth's surface, he added. That means there is enough volcanic material below the surface to match the largest of the supervolcano's three eruptions over the last 2.1 million years, Farrell said. The largest blast - the volcano's first - was 2,000 times the size of the 1980 eruption of Mount St Helens in Washington state. A similar one would spew large amounts of volcanic material in the atmosphere, where it would circle the earth, he said. "It would be a global event," Farrell said. "There would be a lot of destruction and a lot of impacts around the globe." The last Yellowstone eruption happened 640,000 years ago, according to the US Geological Survey. For years, observers tracking earthquake swarms under Yellowstone have warned the caldera is overdue to erupt. Farrell dismissed that notion, saying there isn't enough data to estimate the timing of the next eruption. There are enough instruments monitoring the seismic activity of Yellowstone that scientists would likely know well ahead of time if there was unusual activity, Farrell said.