Some scholars at the University of Utah have drawn up a complete diagram of the plumbing of Yellowstone National Park volcanic system, which is also home to one of the world’s largest volcanoes.

“Really getting an idea of how it works and understanding how these large caldera-forming eruptions may occur, and how they might happen, would be a good thing to understand,”  James Farrell, a geophysicist told the Washington Post.

“No one’s ever witnessed one of these really large volcanic eruptions, Farrell told the newspaper.  “We kind of scale smaller eruptions up to this size and say, ‘This is probably how it happens,’ but we really don’t know that for sure.”

The report fills in a missing link of the system. It describes a large reservoir of hot rock, mostly solid but with some melted rock in the mix, that lies beneath a shallow, already-documented magma chamber, the Washington Post reported.

The newly discovered reservoir is 4.5 times larger than the chamber above it. There’s enough magma there to fill the Grand Canyon, the newspaper reported. The reservoir is on top of a long plume of magma that emerges from deep within the Earth’s mantle.

The report is based on the equivalent of an MRI of the crust beneath Yellowstone. Nature itself supplies the key diagnostic tool: Earthquakes. The Yellowstone region is seismically active, and in any given year there can be hundreds of small earthquakes.

By combining the data from many sensors, scientists can get a picture of the hot and cold rock beneath Yellowstone – also known as “seismic tomography.”