Campi Flegrei does not rise as a single volcanic cone. It is a broad caldera woven into the western suburbs of Naples, the city of Pozzuoli, coastal neighborhoods, and the Bay of Pozzuoli. Roads, homes, rail lines, and archaeological ruins cross ground that has repeatedly moved up and down. That close overlap between an active volcanic system and a dense urban area makes every earthquake swarm and burst of uplift consequential.
The current episode began with renewed uplift in 2005. Seismic activity became more pronounced after 2018, and the system has since produced increasingly felt earthquakes, stronger gas emissions, and periods of rapid ground movement. This behavior is called bradyseism: gradual uplift or subsidence caused by pressure changes beneath a caldera.
A Caldera That Breathes
Campi Flegrei is roughly 12 kilometers across and contains nested structures left by major explosive eruptions. The Campanian Ignimbrite eruption occurred about 39,000 years ago, while the Neapolitan Yellow Tuff eruption followed about 15,000 years ago. Dozens of smaller vents later opened across the caldera. Its most recent eruption built Monte Nuovo in 1538 after years of uplift and several days of intense precursory activity.
That history demonstrates that unrest can culminate in an eruption, but it does not mean every episode will. The caldera underwent major uplift crises in the twentieth century, especially in 1969–1972 and 1982–1984, without erupting. Pozzuoli’s harbor rose enough to disrupt operations, earthquakes damaged buildings, and residents were evacuated. The ground later subsided before the present uplift phase began.
By late July 2026, cumulative uplift near Rione Terra in Pozzuoli had reached about 1.68 meters since January 2011, including roughly 30 centimeters since January 2025. Rates vary through time and can temporarily exceed several centimeters per month. Earthquakes are generally shallow, many occurring within the upper few kilometers, where pressurized rock is breaking.
Magma, Gas, Water, and Weakening Rock
The unrest is not explained by one expanding balloon of magma. Deep magma is the ultimate source of heat and volcanic gases, but the shallow response involves a hydrothermal system filled with hot water, steam, carbon dioxide, and fractured rock. Gas released from magma can rise, heat fluids, and increase pore pressure. As pressure changes, the crust expands, faults slip, and brittle rock fractures in earthquake swarms.
Scientists continue to examine how much of the measured uplift comes from direct magmatic intrusion and how much comes from fluid pressurization. Current official assessments recognize deep magma as the root cause of the crisis but do not find evidence that magma is presently rising toward the surface. That distinction is crucial: a magmatic system can transfer large amounts of gas and heat upward without an eruptible body following the same path.
Repeated deformation also changes the crust itself. Rock that has been stretched, heated, fractured, and chemically altered may become weaker. Consequently, later increments of pressure can produce more earthquakes than earlier ones. Increasing seismicity can therefore reflect both changing pressure and accumulated damage rather than a simple linear march toward eruption.
Why Shallow Earthquakes Matter
Recent swarms have included earthquakes above magnitude 4, strong enough to alarm residents and damage vulnerable buildings. A magnitude 4.6 event in 2025 and a magnitude 4.7 event in July 2026 were among the strongest of the modern crisis. Because many hypocenters are shallow and directly beneath populated areas, shaking can be sharp even when the magnitude is moderate.
The immediate hazard is not limited to a possible eruption. Persistent uplift can strain utilities and transport infrastructure. Repeated shaking can weaken masonry, dislodge nonstructural elements, and make damaged buildings less resilient to the next event. Gas accumulation is another localized concern, especially in poorly ventilated low areas.
Reading the Alert Level Correctly
Campi Flegrei remains under a yellow volcanic alert, signifying sustained unrest that requires close surveillance and preparedness. It is not a prediction that an eruption will occur on a set date. Monitoring networks measure ground movement, earthquake locations, gas composition, temperature, gravity, and other changes. Civil-protection planning separately addresses evacuation routes, communication, structural risk, and the large population within the hazard zone.
Planning Around an Unknown Vent
If an eruption were to occur, its location and size would not be known far in advance. Past activity has opened vents in different parts of the caldera, and many eruptions were far smaller than the events that formed the caldera itself. Hazard planning must therefore account for uncertainty rather than focus solely on a worst-case “supereruption.”
Campi Flegrei’s unrest is real, prolonged, and disruptive. Yet the evidence supports a careful statement, not a countdown: pressure is changing within a coupled magmatic-hydrothermal system, shallow crust is fracturing, and scientists have not detected proof of magma ascending toward eruption. Living with the caldera means taking those observations seriously while preserving the difference between elevated hazard and certain catastrophe.