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. Vulcanologists called for existing evacuation plans, which assume that an eruption can be predicted 72 hours in advance, to be updated to include the possibility of having to evacuate all those people after an eruption has already begun.
On October 31 the Italian minister for civil protection, Nello Musumeci, said he would consider raising the alarm level if the seismic activity continued—a move that would activate the evacuation of certain hospitals and prisons and encourage the most immediate 500,000 residents to voluntarily leave their homes. After a few quieter weeks, another swarm of tremors shook the area on November 23, prompting media to again speculate about the likelihood of magma rising to the surface.
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.
Tension among residents has remained high because scientists don’t know for sure what’s going on under the surface. 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.
After its most recent eruption in 1538 the volcano went quiet. By now, “any previous connection between the molten rock underground and the surface has been sealed up,” says Christopher Kilburn, a volcanology and geophysical hazards professor at University College London. As a result, the crust serves as a barrier, and Kilburn says that before another eruption can take place, the crust has to be ruptured, creating a new pathway for lava or fluids to breach.
Scientists think that has been happening since Campi Flegrei awakened in the 1950s. At that time modest seismicity picked up, paired with the slow flexing—uplift and sinking—of the ground. Scientists say pressure from below the top part of Earth’s crust pushes against it at a depth of two to three km, causing it to stretch and fracture, creating superficial earthquakes and the surface bulge. Between 1982 and 1984 the ground rose 1.8 meters, and some 30,000 people were evacuated in what many scientists consider an aborted eruption—magma is thought to have ascended close to the surface, only for something to then halt its rise. The ground began deflating again until 2004, when the current uplift began.
According to a 2023 paper co-authored by Kilburn in Communications Earth & Environment, each uplift episode stretches the crust further, creating conditions more favorable to a rupture and opening a pathway for an eruption.
But that’s where the division lies. According to the shallow magma model, rising magma is piling pressure on the crust, which happened when the ground rose in the 1980s. According to the hot fluids model, which has gained more traction since seismicity picked up in the area in 2016, the magma sits deeper, but it is sending larger and larger quantities of steam and hot gases toward the surface.
Finding definitive evidence for either model remains elusive. Geophysicists lack direct access to the complex underground phenomena that they study. Instead they analyze the indirect signals of those processes that reach the surface, such as seismicity, ground uplift and gases emitted by vents called fumaroles. “That’s not unique to Campi Flegrei,” Kilburn says. “Whenever a volcano reawakens, we all have to use a little bit of imagination to work out what the signals mean.” Moretti likens the efforts of volcanologists to those of the physicians of the past who tried to discern human diseases only from a person’s symptoms, without having detailed knowledge of internal bodily processes.
Moretti and Kilburn, among others, note that the uplift has so far been slower than in 1982–1984, when it was likely caused by magma rising at shallow depth, which hasn’t really been seen in the current circumstances. The hot fluids model would be consistent with the large quantities of carbon dioxide measured at fumaroles and the shape of the ground’s bulge, which is greater at the epicenter, close to the town of Pozzuoli, Italy, and gradually decreases from there.
The one investigation scientists could conduct is to drill. “Boreholes are the most efficient and direct way to study geology and volcanoes,” says Giuseppe De Natale, research director at Italy’s National Institute of Geophysics and Volcanology, who agreed to speak to Scientific American as an individual researcher, not a representative of the institute. De Natale led efforts to drill a 500-meter pilot borehole in 2012 that provided scientists with more precise stratigraphical information about the origins and boundaries of the caldera. But he says that local politicians and media, together with a local scientist, began to “wage war” on the project by describing it as dangerous. Public opinion turned against a second, 3.5-km borehole, causing funders to pull their support.
It’s unclear whether support for new boreholes has increased now that the threat seems greater. De Natale says news of new drilling would likely cause a similar reaction, so right now drilling initiatives have been shelved. A 3.5-km borehole would take about a month to drill. It would have a diameter between 30 and 35 centimeters close to the surface and 10 to 12 cm at deeper levels. One such borehole would inevitably pierce Earth’s crust, but De Natale says that would pose few risks to local residents because modern boreholes are equipped with blowout preventers—mechanical devices also used in oil wells that monitor and seal the boreholes when pressure exceeds a certain threshold. Moretti says drilling could generate seismicity, and hot, acidic fluids could spurt out—as they do in geysers.
Boreholes would allow scientists to study deep geochemical compounds, as well as rocks, including their temperature and pressure. Additionally, boreholes would help researchers understand how much more the crust can stretch. “We know that the ground rose four meters since 1950 and 1.17 meters since 2005—but we don’t know how much more pressure the rocks can bear,” De Natale says. Four meters of uplift could be moderate, he says, or it could be close to the critical point of an imminent eruption.
Kilburn says the differences among scientists could sound like nitpicking to people on the caldera, because as long as the uplift continues, pressure underground will build, and seismicity will continue to increase. Yet De Natale says stronger earthquakes could also mean that fractures are taking place underground, allowing some of the pressure to ease. A similar trend appears to have occurred at the end of October, when the number of tremors under Campi Flegrei decreased. But De Natale says the trend might be short-lived: “Fractures heal over time, and when they close, pressure begins building again,” he says. Something similar happened in 2013, when seismicity appeared to drop only to pick up after a year. “It seemed as if it were all over,” De Natale adds. “But it all started again, just like before.”