Unsettling new research shows it could take only decades for the giant Yellowstone volcano to reawaken, plunging the world into a volcanic winter and potentially destroying all humanity.
The last time Yellowstone erupted - 631,000 years ago - it shot out 1,000 cubic kilometres of rock and ash, choking the sky and creating a 72km-wide caldera that still exists today.
Researchers recently studied the fossilised ash deposit from its last supereruption. Inside, they tracked the changes that the volcano went through before its eruption.
The new evidence shows that Yellowstone's last eruption came after mere decades of magma build-up. Previously, geologists had thought it took millennia.
"It's shocking how little time is required to take a volcanic system from being quiet and sitting there to the edge of an eruption," researcher Hannah Shamloo told the New York Times.
Scientists hope to use the research to predict when future supereruptions might occur - allowing enough time to develop technologies to prevent the disaster.
The US Geological Survey puts the yearly odds of another giant Yellowstone blast at around one in 730,000.