About 630,000 years ago, a powerful eruption took place in what is now Wyoming. This explosion spewed 240 cubic miles of rock and ash, creating the Yellowstone caldera—a volcanic depression spanning most of the national park. The supervolcano under Yellowstone has been mostly dormant for tens of thousands of years—the last large eruption filled the caldera with ash and lava around 70,000 years ago, well before people had migrated to the continent.

New research from Arizona State University shows that the last time the volcano erupted was after two large influxes of magma filled the reservoir under the caldera—and that process may take less time than previously thought. The current theory has its origins in a 2013 study that concluded the reservoir is 2.5 times larger than previous estimates, and since it drains after every massive explosion, geologists thought it would take a long time to refill. But when scientists analyzed the minerals in fossilized ash from the most recent mega-eruption, they found that critical changes in temperature and composition happened in a matter of decades, not centuries.

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Yet a massive eruption in the middle of the U.S. is still an unlikely event. Here’s Victoria Jaggard reporting for National Geographic:

But almost everyone who studies Yellowstone’s slumbering supervolcano says that right now, we have no way of knowing when the next big blast will happen. For its part, the U.S. Geological Survey puts the rough yearly odds of another massive Yellowstone blast at 1 in 730,000—about the same chance as a catastrophic asteroid collision.

Yellowstone is one of the most closely watched volcanoes in the world. There is a whole suite of sensors and satellites that track any and all detectable changes. For now, at least, geologists aren’t terribly concerned.

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Watch a preview for NOVA’s “Killer Volcanoes,” airing October 25, 2017 at 9 pm on PBS. Scientists hunt for a mega-eruption that plunged medieval Earth into a deep freeze: