Sitting within the Bay of Naples in southern Italy is Campi Flegrei, a vast and restless volcanic caldron. The history of this sleeping colossus includes two massive eruptions, 39,000 and 15,000 years ago, that left deep calderas in the landscape. Its last significant volcanic event was a 1538 eruption known as Monte Nuovo that spawned a small n ew mountain. Since then, it has been curiously eruption-free.

Today, 1.5 million people live within the volcano ’s caldera and its surroundings, and the cache of magma that could burst from the area’s hellish underbelly makes it one of the most hazardous areas on Earth. In a study published Wednesday in Science Advances, volcanologists report that Campi Flegrei is at the start of an eruption cycle, one that may result in a massive outburst at some point in the likely distant future.

The researchers arrived at this conclusion by unspooling the 60,000-year-long history of the volcano, which revealed what appears to be a rhythm to Campi Flegrei’s upsurges. Fortunately, there are no signs that an eruption is imminent. And because the volcano is one of the world’s most closely monitored, scientists are likely to pick up on any warning signs.

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Previous studies of Campi Flegrei have focused on one or a handful of its eruptions. But the new study, led by Francesca Forni, a postdoctoral researcher at Nanyang Technological University in Singapore, who conducted the work at ETH Zurich, used fresh geological samples taken from 23 eruptions across its history, both larger and smaller , to see how the chemistry of the magma changed over deep time. Her team also used computer models to simulate what may have occurred inside the volcano since the last caldera-forming eruption 15,000 years ago.