On the night of October 2 a worrisome earthquake shook the towns above Campi Flegrei, a huge, long-dormant volcano immediately west of Naples, whipping local media and authorities into a frenzy. Journalists speculated that lava might start flowing from the volcano, threatening the 1.3 million people who live in high-risk areas near its center.

The October 2 earthquake, which was magnitude 4.0, and the hundreds more since then have capped the most intense period of seismic activity that Campi Flegrei has exhibited in decades. In the two months before October 2, more than 2,000 low-magnitude tremors were recorded in the region, including the strongest quake since 1983. Monitoring systems showed that the ground in some places had risen by 1.17 meters since 2005, and two thirds of that had happened since 2016.

The scientific community agrees that the tremors and uplift are signs that the volcano is awakening. But they are struggling to rectify two competing explanations for the bulging ground that have been debated for decades, leaving residents and researchers uneasy. An answer to the geological mystery could bring scientists much closer to determining how likely the volcano is to blow. It could also provide geologists worldwide with warning signs they could look for when other big volcanoes start rumbling, especially supervolcanoes such as Yellowstone in the northwestern U.S., Toba in Indonesia and the Altiplano-Puna volcanic complex in Argentina, Bolivia and Chile.

One model—let’s call it the shallow magma model—posits that the seismicity and bulging are caused by magma pushing to break through the surface, making an explosive eruption, with violent magma outflow highly likely in the near term. Alternatively, in the hot fluids model, steam and hot gases released by magma located deeper underground are to blame. In that case, the ongoing seismic activity could stop abruptly or peak in a phreatic eruption—the volcano would spurt out hot liquids, gases and rock fragments instead of lava. This would pose a lesser threat, although it would still be a lethal one because so many people live close by.

“Everyone agrees that magma is involved,” says Roberto Moretti, an associate professor of geochemistry and volcanology at the University of Campania Luigi Vanvitelli in Italy, and a proponent of the hot fluids model since 2013. But scientists disagree on what role magma plays exactly and consequently how close it is to the surface. “Hence the big question,” Moretti says. “Where is the magma?”

The volcano, known as Phlegrean Fields in English, comprises two dozen craters and other structures in an area 14 kilometers across. One third of it lies under the Tyrrhenian Sea, between the Italian mainland and the country’s island of Sardinia. The volcano has been active for at least 80,000 years. Its caldera—the depression created when emptying magma chambers cause the roof of a volcano to collapse—formed after two violent eruptions 39,000 and 15,000 years ago. The older one caused a volcanic winter in areas within 100 km, sent ash as far away as Russia and abruptly cooled the climate around the world.