A big reservoir of hot, partly molten rock has been discovered beneath the famed Yellowstone National Park in the United States, but researchers said Thursday there is no added risk of volcanic eruption.

The findings in the journal Science show for the first time that the amount of magma beneath the surface is far bigger than previously thought.

The reservoir lies 12 to 28 miles (19 to 45 kilometers) beneath the Yellowstone supervolcano and is more than four times bigger than the magma chamber that is already known to exist.

"For the first time, we have imaged the continuous volcanic plumbing system under Yellowstone," said co-author Hsin-Hua Huang, a post-doctoral researcher in geology and geophysics at the University of Utah.

"That includes the upper crustal magma chamber we have seen previously plus a lower crustal magma reservoir that has never been imaged before and that connects the upper chamber to the Yellowstone hotspot plume below."

Experts say there is one in 700,000 annual chance of a volcanic eruption at Yellowstone, which spans the midwestern US states of Wyoming, Idaho and Montana.

The Yellowstone supervolcano last erupted about 640,000 years ago.

"The magma chamber and reservoir are not getting any bigger than they have been, it's just that we can see them better now using new techniques," said co-author Jamie Farrell, also a post-doctoral researcher at the University of Utah.