Enough hot rock sits beneath Yellowstone National Park to fill the Grand Canyon nearly 14 times over, according to our best view yet of the supervolcano that lies below the famous landscape.

The first three-dimensional image of the inner workings of the Yellowstone supervolcano has revealed an 11,200-cubic-mile magma reservoir about 28 miles below the surface. A previously known 2,500-cubic-mile magma chamber sits above that, at about 12 miles deep. Both serve as conduits between a hotspot plume that may originate in the Earth’s core and the Yellowstone caldera at the planet’s surface.

“Every additional thing we learn about the Yellowstone volcanic system is one more piece in the puzzle, and that gets us closer to really understanding how the volcanic system works,” says study co-author Fan-Chi Lin of the University of Utah. “If we could better understand the transport properties of magmatic fluids, we could get a better understanding of the timing and, therefore, where we are in the volcanic cycle.”

The Yellowstone hotspot plume has been producing eruptions for the last 17 million years. Due to plate tectonics, Earth’s surface has moved over the hotspot, creating a track of ancient eruptions that stretches from the Oregon-Idaho-Nevada border—the site of the first eruption—to the Yellowstone caldera. Since the hotspot reached Yellowstone some 2 million years ago, the supervolcano has erupted three times, most recently about 640,000 years ago.

The hotspot currently feeds the geysers, hot springs and steam vents that are part of the draw of Yellowstone National Park. The chance that the supervolcano will erupt anytime soon is low—only about 1 in 700,000 annually. But should there be another eruption, the supervolcano could spew some 640 cubic miles of debris, covering large swathes of North America in ash and darkening skies for days.

This animation shows the volcanic plumbing system of the Yellowstone supervolcano, including a newfound magma reservoir (red) about 28 miles below the surface. The green lines represent the boundary of Yellowstone National Park. (Credit: Hsin-Hua Huang, University of Utah Department of Geology and Geophysics)

In the new study, appearing today in Science, a team led by Hsin-Hua Huang of the University of Utah used earthquake data to create the equivalent of a CT scan of the Yellowstone system. The density of the Earth affects how fast seismic waves travel through it, while the temperature of the rock affects density. By combining data from thousands of earthquakes recorded both locally and across the United States, the researchers created a 3-D map of what is happening deep underground. That revealed the new magma reservoir, which had been previously suspected but unconfirmed until now.

Neither the reservoir nor the upper magma chamber is a seething cauldron of hot magma. Both are regions of hot rock with a small percent of molten rock—about 9 percent in the upper chamber and a mere 2 percent in the lower reservoir. The two regions are probably connected by a system of volcanic dikes and sills, Lin says.

Because researchers are currently unable to see how that molten rock is situated within the two chambers, “it is hard to infer anything about a future eruption,” Lin says. However, it is likely that there would be many warning signs of such an occurrence, such as increased amounts of earthquake activity, earthquake swarms, ground uplift, increases in the amount of volcanic gasses coming out of the ground and trees killed by higher ground temperatures. “We can’t say how much of a warning we would have,” he says, “but it could be on the order of days to months to years.”