The magma reservoir under the Yellowstone National Park is 50 miles (80km) long and 12 miles (20km) wide: more than two and a half times larger than expected. Nature describes the reservoir as "a 4,000 cubic kilometer underground sponge," between six and eight percent of it filled with molten rock.

Nature reports the imaged magma blob is three to six miles below the earth's crust, and separated from the surface by "cooler and more brittle" rocks that fracture easily under geomorphic pressure. Postdoctoral researcher at the University of Utah Jamie Farrell made the discovery after studying data recorded from more than 4,500 earthquakes. Yellowstone is one of the world's most geologically active places: Farrell and his Utah team noted small quakes occurring "as often as every few seconds."

Volcanoes that spew the underground magma to the surface as lava are rare. The last monster eruption took place some 640,000 years ago; the most recent smaller eruption was about 70,000 years ago. Earthquakes measuring over 7 on the Richter scale have occurred in Yellowstone this century: 1959's Hebgen Lake earthquake measured 7.3, and killed 28 people.

Yellowstone's magma reservoir is one of the world's largest. Geophysicist Robert Smith — also on the Utah-based team — said he didn't know "of any other magma body that's been imaged that's that big." But Farrell and other scientists say that despite the vast vat of magma and the ground under the park's supervolcano "bulging" in 2011, Yellowstone's main threat remains earthquakes.