Scientists say a mammoth magma reservoir lies under Yellowstone National Park. It's four times the size of the magma pool that fuels the supervolcano.

It holds enough hot rock to fill the Grand Canyon 11 times, but scientist say you shouldn't worry -- it's not likely to erupt soon.

The reason Yellowstone has the most and largest geysers, pools and other thermals on earth is because of a mammoth magma system underneath it, which also fueled several huge volcanic eruptions over millions of years.

"This is the one big continental hot spot of the world," Dr. Robert Smith, a professor at the University of Utah, said.

Smith is considered the world's foremost expert on the Yellowstone supervolcano system. His publication last year drew attention worldwide, when it declared the magma body under the park was two and a half times larger than previously thought.

"It's 90 kilometers long,” Smith said. “It begins southwest of Old Faithful. Goes way beyond the caldera."

His newest publication shows a much, much larger magma reservoir below the one previously mapped.

"This lower crustal body we just imaged is four times bigger than the shallow one right at the surface,” he said.

Yellowstone geologist Dr. Henry Hessler says Smith's research confirms what geologists suspected for more than 100 years.

MORE>>>NBC