West of Naples, beneath one of the most densely settled coastlines in Europe, lies a volcano that does not look like one. Campi Flegrei — the Phlegraean Fields, or “burning fields” — is not a single cone but a sprawling caldera roughly eight miles across, a collapsed volcanic basin whose center sits under the seaside city of Pozzuoli and whose rim reaches into the suburbs of Naples. For years the ground here has been slowly rising and shaking, a restless behavior that has put scientists on alert and unsettled the hundreds of thousands of people who live directly on top of the system.
What makes Campi Flegrei so closely watched is the combination of a large, potentially violent volcanic system and an enormous population living within its most hazardous zone. Unlike a mountain that erupts from a distant summit, this caldera’s danger is spread beneath homes, roads and ancient ruins, which means even modest volcanic unrest translates directly into risk for a major urban area.
The bradyseism that lifts and drops the ground
The defining feature of Campi Flegrei is a phenomenon called bradyseism, the gradual uplift and subsidence of the ground as pressure changes in the shallow crust beneath the caldera. The land has been rising in fits and starts since the mid-2000s, and the rate has generally accelerated over time. According to the Phlegraean Fields caldera, the pattern has repeated across recorded history, most dramatically in the 1970s and 1980s, when uplift damaged parts of Pozzuoli and forced temporary evacuations of the old harbor district.
The current episode has been substantial. Monitoring by Italy’s National Institute of Geophysics and Volcanology, known as INGV, tracked total ground uplift in Pozzuoli reaching about 1.33 meters — more than four feet — by late 2024, measured against a network of benchmarks maintained by the institute’s Vesuvius Observatory. As the ground swells, it fractures, and those fractures are what generate the earthquakes that residents feel.
Thousands of quakes shaking Pozzuoli
The seismicity has been persistent rather than catastrophic, but its sheer volume is striking. Since 2020, the caldera has produced tens of thousands of shallow earthquakes, most too small to cause damage, with the strongest reaching into the magnitude 4 range. The tremors are so shallow — typically only a couple of miles beneath the surface — that even modest quakes can shake buildings hard, and repeated shaking has cracked walls and rattled nerves in Pozzuoli and neighboring parts of Naples.
Researchers at INGV have worked to quantify the link between the uplift and the quakes, showing how the swelling ground drives the seismic activity as stresses build in the rock. Their analysis of the caldera’s behavior underscores that the unrest is a coupled system: as long as the ground keeps rising, the earthquakes are likely to continue. That relationship is central to interpreting whether the volcano is merely venting pressure or building toward something more serious.
Why scientists debate what is driving the unrest
The most important open question is what lies beneath the swelling: rising magma, or hot fluids and gases moving through the crust. The distinction matters enormously, because a pulse of new magma approaching the surface would be far more alarming than the circulation of superheated water and volcanic gas. Modeling of the deformation data has suggested that magma has risen from deep within the system over recent decades, feeding a shallower reservoir, while a large volume of gas released from that magma pressurizes the rock closer to the surface and helps drive the uplift and quakes.
That ambiguity is why Campi Flegrei is monitored so intensively, with continuous GPS, seismic, gas and ground-temperature measurements feeding into hazard assessments. The caldera’s history sets the stakes: it produced two of the largest eruptions in the Mediterranean region tens of thousands of years ago, and its most recent eruption, in 1538, built a new hill called Monte Nuovo in about a week. Even a comparatively small eruption today would unfold in the middle of a metropolitan area.
Half a million people inside the red zone
The human dimension is what elevates Campi Flegrei from a scientific curiosity to a civil-protection challenge. Italian authorities have designated a “red zone” around the caldera where the risk of pyroclastic flows in an eruption is highest, and roughly half a million people live within it. The national emergency plan spans Pozzuoli, Bacoli, Monte di Procida and Quarto, along with parts of Naples and other neighboring municipalities, and it calls for the preventive relocation of red-zone residents within 72 hours if monitoring indicates a shift to the highest alert level.
Evacuating a population that size from a congested coastal area within three days is a formidable logistical problem, and Italian officials have run drills and updated planning to prepare for it. The strong earthquakes of recent years have also prompted more immediate measures, from building inspections to public information campaigns, as the region tries to balance the low year-to-year probability of an eruption against the extraordinary consequences if one occurred. For now, Campi Flegrei remains in a state of unrest rather than eruption, its ground rising and its earth trembling beneath one of the most heavily populated volcanic settings on the planet — a reminder that some of the world’s most dangerous volcanoes are also the ones people have chosen to live directly on top of.
This article was produced with AI assistance and reviewed by Morning Overview editors.
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