The ground near Naples is bulging again as an ancient supervolcano stirs

West of Naples, one of the world’s largest calderas has been swelling again, pushing the ground upward and rattling nearby neighborhoods with clusters of small earthquakes. Campi Flegrei, known in English as the Phlegraean Fields, sits partly beneath the Bay of Pozzuoli and partly under some of the densest suburbs of the Naples metropolitan area, putting hundreds of thousands of residents directly on top of an active volcanic system. The current phase of unrest has been building for roughly two decades, and readings through 2026 have been among the most closely watched in that stretch.

The prehistoric eruptions that carved the caldera

Campi Flegrei’s basin shape is itself the product of two enormous prehistoric eruptions rather than a single volcanic cone. The Campanian Ignimbrite eruption, roughly 39,000 years ago, ranks among the largest volcanic events in Europe over the last quarter-million years, spreading ash across much of the Mediterranean and into southern and eastern Europe. A second, smaller collapse, the Neapolitan Yellow Tuff eruption around 15,000 years ago, carved out the roughly 12-to-13-kilometer-wide depression that gives the caldera its modern outline and that now cradles Pozzuoli, the western edge of Naples, and the Bay of Pozzuoli itself. Those two events emptied enough magma from the shallow reservoir beneath the region to cause the ground above to collapse inward, which is why volcanologists classify Campi Flegrei as a true caldera rather than a stratovolcano like Vesuvius, its more famous neighbor across the bay. Italy’s civil protection system tracks the ongoing unrest against that deep geological history using a four-level color scale that moves from a green base condition through yellow attention and orange pre-alarm to red alarm, with each threshold defined by combined readings of uplift rate, seismic energy, and gas chemistry rather than any single number.

Bradyseism, the slow heartbeat of the caldera

The phenomenon driving the concern is called bradyseism, a term for the gradual rise and fall of ground surface caused by pressure changes in the hot fluids and gases trapped beneath a caldera. Campi Flegrei has cycled through bradyseism episodes for centuries, a pattern first documented by Roman-era ruins at the nearby Macellum of Pozzuoli, where marine mollusk borings on marble columns show the ground has both sunk below sea level and risen well above it over time. The current episode began accelerating in the early 2000s and has continued, with periods of faster and slower uplift, into the present decade, according to the Wikipedia entry documenting the caldera’s history and monitoring record.

What the ground has done at Pozzuoli

The point of maximum deformation sits at Rione Terra, the old town center of Pozzuoli, where cumulative uplift since the unrest began has climbed past a meter and a half. The rate has not been steady. Monitoring bulletins tracked a stretch of faster lifting in late 2025, followed by a slowdown through the first half of 2026, alongside swarms of mostly minor earthquakes, the great majority below magnitude 3, with occasional stronger shocks that are felt across Pozzuoli and parts of western Naples. Italy’s civil protection authority keeps a running public account of the unrest, including current alert levels and evacuation planning for the area, on its dedicated bradyseism monitoring page.

Why INGV says an eruption is not imminent

Italy’s National Institute of Geophysics and Volcanology, INGV, has spent years quantifying the relationship between the ground uplift and the earthquake swarms that accompany it, work aimed at distinguishing routine bradyseism from a buildup that could precede an eruption. Researchers there have concluded that present conditions do not support an eruptive event, a finding tied to how deep the pressurized source sits and how much magma or hydrothermal fluid would need to accumulate to change that picture. INGV has also noted that only a continuation of uplift at rates comparable to today’s, sustained for decades, would begin to approach the scale of material involved in the caldera’s last eruption, detailed in the institute’s press release on quantifying uplift and seismicity.

The 1538 eruption still shapes today’s planning

That last eruption, in 1538, built the cone now known as Monte Nuovo in a matter of days, following a bradyseism episode that lifted parts of the coastline dramatically before the vent opened. It remains the benchmark against which scientists measure the current unrest, both because it is the caldera’s most recent eruptive event and because it demonstrated how quickly a bradyseism crisis can, in principle, culminate in an eruption once enough uplift and seismic energy accumulate. Modern evacuation planning for Campi Flegrei is built around that history, with tiered alert levels that would trigger phased evacuations of the roughly 500,000 people living inside the caldera’s boundary well before any eruption became likely, rather than waiting for a single dramatic warning sign.

A caldera scientists are used to watching, not predicting

Volcanologists are careful to separate the two things people often conflate when they hear about a “supervolcano” stirring: continuous, well-instrumented monitoring, which Campi Flegrei has in abundance, and any specific forecast of when or whether an eruption will occur, which the current science does not support. The caldera is one of the most heavily monitored volcanic systems on Earth, with GPS stations, tiltmeters, gas sensors, and seismic networks feeding data to INGV around the clock. That density of instrumentation is precisely why small changes in uplift rate or earthquake frequency make headlines: scientists can see the caldera breathing in far more detail than they can at almost any comparable system, even though the fundamental question of what happens next remains, as it has for two decades, unresolved.

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

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