A new study shows the chamber of hot molten rock below Yellowstone National Park is more than 2½ times larger than previously estimated.
Lead author Jamie Farrell of the University of Utah said Monday the magma chamber is about 55 miles (88.5 kilometres) long, 29km wide and runs at depths from 5 to 14½km below the earth.
That means the supervolcano below Yellowstone could erupt with the force of its largest-ever eruption 2.1 million years ago.
Farrell says that eruption was about 2000 times the size of Mount St Helens in 1980, and a similar explosion would be a "global event".
Farrell says there have been three eruptions of the supervolcano, and scientists don't know when the next will happen.