Scientists mapped the magma chamber underneath the Yellowstone caldera by measuring seismic waves from earthquakes. They found that the hot molten rock is 55 miles long - which is about 2 1/2 times larger than previously estimated.
The chamber is 18 miles wide and runs at depths from 3 to 9 miles below the earth, lead author Jamie Farrell of the University of Utah said Monday.
This means that there is enough volcanic material below the surface to match the largest of the supervolcano's three eruptions over the last 2.1 million years, Farrell said.
The largest blast was the volcano's first. It was 2,000 times the size of the 1980 eruption of Mount St. Helens in Washington state. A similar one would spew large amounts of volcanic material in the atmosphere, where it would circle the earth, he said.
"It would be a global event," Farrell said. "There would be a lot of destruction and a lot of impacts around the globe."
According to the U.S. Geological Survey, the last Yellowstone eruption happened 640,000 years ago. For years, observers tracking earthquake swarms under Yellowstone have warned the caldera is overdue to erupt.
Farrell says there isn't enough data to estimate the timing of the next eruption. He did say, however, that there are enough instruments monitoring the seismic activity of Yellowstone that scientists would likely know well ahead of time if there was unusual activity happening and magma was moving to the surface.
The USGS' Yellowstone Volcano Observatory listed the park's volcano alert level as "normal" for December.
Farrell presented his findings last week to the American Geophysical Union. He said he is submitting it to a scholarly journal for peer review and publication.
Yellowstone attracts millions of visitors with its geothermal features of geysers, hot springs and bubbling mud pots. The park just opened its gates on Sunday for its winter season.
The Associated Press contributed to this report.