A new study done on ancient volcanic ash revealed that we may experience an eruption of the Yellowstone supervolcano even sooner than previously warned. Scientists are also concerned that we will probably have much less advance warning time than we had thought before.

According to National Geographic, we may have mere decades before Yellowstone erupts. If the supervolcano underneath Yellowstone national park erupts again, we could also have far less time to prepare than originally thought. After analyzing minerals in fossilized ash from the most recent mega-eruption, researchers at Arizona State University think the supervolcano last woke up after two influxes of fresh magma flowed into the reservoir below the caldera. The new paper adds to the lengthening list of surprises scientists have uncovered over the last few years as they have continued to study and closely monitor the supervolcano.

A 2013 study, for instance, showed that the magma reservoir that feeds the supervolcano is about two and a half times larger than previous estimates. Scientists also think the reservoir is drained after every monster blast, so they thought it should take a long time to refill. Based on the new study, it seems the magma can rapidly refresh—making the volcano potentially explosive in the geologic blink of an eye. –National Geographic

“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,” study co-author Hannah Shamloo told the New York Times. But scientists insist that this may seem scary, however, it may still be awhile before the eruption occurs.

About 630,000 years ago, a powerful eruption shook the region, spewing forth 240 cubic miles’ worth of rock and ash. The violent eruption created the Yellowstone caldera, a volcanic depression 40 miles wide that now cradles most of the national park. That eruption also left behind the Lava Creek Tuff, the ash deposit that Shamloo and her ASU colleague Christy Till used for their work. The two scientists presented their findings in August at a volcanology meeting in Oregon. The pair also presented an earlier version of their study at a 2016 meeting of the American Geophysical Union.

Based on fossil deposits like this one, scientists believe that the supervolcano has seen at least two other eruptions on this scale in the past two million years or so. 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. That’s about the same chance as a catastrophic asteroid collision with earth.

Yellowstone is one of the best-monitored volcanoes in the world, notes Michael Poland, the current Scientist-in-Charge of the Yellowstone Volcano Observatory for the U.S. Geological Survey. A variety of sensors and satellites are always looking for changes, and right now, the supervolcano does not seem to pose a threat. “We see interesting things all the time … but we haven’t seen anything that would lead us to believe that the sort of magmatic event described by the researchers is happening,” says Poland via email, adding that the research overall is “somewhat preliminary, but quite tantalizing.”