A restless supervolcano under a Italian city keeps lifting the ground beneath homes

West of central Naples, a low volcanic landscape of craters, hot springs and steaming vents spreads across the Gulf of Pozzuoli, largely hidden inside an ordinary Italian cityscape of apartment blocks and harbor roads. It has not erupted since 1538, but for two decades the ground beneath entire neighborhoods has been slowly rising, and Italian authorities now track the unrest through a formal alert system built specifically for this one volcano.

A Caldera Carved by Two of Europe’s Largest Eruptions

The Campi Flegrei volcanic system, whose name means “burning fields,” covers an area of roughly 130 square kilometers including the part submerged beneath the gulf, according to Italy’s national geophysics institute, INGV. Its eruptive history began more than 80,000 years ago and is dominated by two enormous explosive eruptions: the Campanian Ignimbrite, roughly 40,000 years ago, considered the most violent eruption in the Mediterranean’s history, which buried two-thirds of the surrounding Campania region under a thick blanket of volcanic material and triggered the ground collapse that formed the original caldera, later invaded by the sea. That single event dwarfs the far more famous 79 A.D. eruption of Vesuvius, the volcano visible across the Bay of Naples to the east. A second eruption, the Neapolitan Yellow Tuff, followed roughly 15,000 years ago, devastating an area of about 1,000 square kilometers and causing a smaller caldera to sink within the first. Over the following 10,000 years, the center of that inner caldera rose back up by about 90 meters in a process geologists call resurgence, a slow uplift that continues to shape the area’s geology today.

More Than 70 Eruptions, and a Last One in 1538

Volcanic activity continued inside the caldera after the Yellow Tuff eruption, producing more than 70 additional eruptions concentrated into distinct active periods separated by long stretches of quiet, spanning roughly 15,000 to 3,800 years ago. The most recent eruption, in September 1538, built the cinder cone now known as Monte Nuovo. INGV records show that event was preceded by clear warning signs, including increased earthquake activity, ground uplift at the site of the forming crater, and gas emissions, before the eruption itself unfolded over about a week, dominated by explosive interactions between magma and water. The eruption destroyed the village of Tripergole, along with its churches, thermal baths and other buildings, and significantly reduced the size of nearby Lake Lucrino under a blanket of ejected material. Today the area is quiescent in eruptive terms, but its fumarole fields at Solfatara and Pisciarelli remain visibly and continuously active, releasing steam and gas from the shallow hydrothermal system beneath the surface.

Two Decades of Rising Ground Under Pozzuoli

Campi Flegrei’s ground has moved up and down through documented episodes since Roman times, a phenomenon called bradyseism, with the most recent major swings in 1969-72 and 1982-84 forcing many residents of Pozzuoli’s historic center to abandon their homes. A new uplift phase began in 2005 and was still underway as of April 2025, when INGV recorded a cumulative rise of about 144 centimeters at its measurement point in Pozzuoli. That uplift has been accompanied by a rise in earthquake activity, including some of the strongest tremors the area has recorded in 40 years, among them a magnitude 4.4 event on May 20, 2024, and a magnitude 4.6 event on March 13, 2025. INGV’s monitoring networks show the uplift forming a roughly circular pattern centered just southeast of the Rione Terra district of Pozzuoli, with a rate that has varied over the years between about 10 and 30 millimeters per month. That range matters because the pace of uplift, not just its total amount, is one of the signals INGV scientists track when judging whether unrest is stable, accelerating or slowing, alongside the number and depth of earthquakes and changes in the temperature and pressure of the hydrothermal fluids feeding the area’s fumaroles.

A Six-Level Alert System Built for This Kind of Unrest

Because Campi Flegrei’s hazard profile does not resemble a classic cone volcano, Italy’s Department of Civil Protection maintains a dedicated alert scale specific to the caldera, formalized in an October 30, 2025 decree covering six escalating levels: green for quiescent conditions, two tiers of yellow covering low and medium unrest, two tiers of orange covering high and very high unrest, and red for a pre-eruptive state. The area has stood at yellow since 2012, a status INGV’s own page describes simply as “attention,” corresponding to monitoring parameters that remain above baseline but have not reached the thresholds tied to a medium-to-high probability of eruption. Each level carries its own list of possible hazards, ranging from localized carbon dioxide buildup in low-lying areas at green and yellow, to a heightened chance of phreatic steam explosions and seismic-triggered landslides at orange, reflecting a system designed to track gradual escalation rather than to issue a single yes-or-no eruption warning.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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