You know that supervolcano in Yellowstone National Park? The one that, three times in the last 2 million years, spewed enormous amounts of ash over the North American continent? Scientists have discovered an enormous underground reservoir deep beneath the surface and have mapped it out for the first time.

Don’t worry. There’s not a lot of actual molten rock in there and it doesn’t at all affect the likelihood of whether it will erupt anytime soon – the odds each year are still roughly 1 in 700,000. But the findings published online by the journal Science provide deeper (so to speak) insight on this mysterious supervolcano sitting in our backyard and on the inner workings of other supervolcanoes around the world.

“Now we really have a complete image of the Yellowstone plumbing system,” study co-author Jamie Farrell, a geophysicist at the University of Utah in Salt Lake City, said in an interview.

The Yellowstone caldera, a giant crater caused by a previous eruption, measures 40 miles by 25 miles and sits in the northwest corner of Wyoming, in Yellowstone National Park. The supervolcano erupted 2 million, 1.2 million and 640,000 years ago, fed by the movement of the North American tectonic plate. Underground, 3 to 9 miles beneath the caldera, sits a frying-pan-shaped magma chamber measuring roughly 19 miles by 55 miles.

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