A team found the cavern stretches for more than 90km (55 miles) and contains 200-600 cubic km of molten rock.
The findings are being presented at the American Geophysical Union Fall Meeting in San Francisco.
Prof Bob Smith, from the University of Utah, said: “We’ve been working there for a long time, and we’ve always thought it would be bigger... but this finding is astounding."
If the Yellowstone supervolcano were to blow today, the consequences would be catastrophic.
The last major eruption, which occurred 640,000 years ago, sent ash across the whole of North America, affecting the planet’s climate.