A restless supervolcano under Naples has bulged upward beneath 500,000 people

West of Naples, the ground itself has been rising. Beneath the densely built towns that ring the bay sits Campi Flegrei, a sprawling volcanic caldera whose floor has been swelling upward for years, lifting streets, harbors and homes a few centimeters at a time and rattling the region with swarms of small earthquakes.

The caldera is one of the most closely watched volcanic systems on Earth precisely because of who lives on top of it. Hundreds of thousands of people occupy the area inside and around the crater, and the slow, grinding deformation — along with the tremors that come with it — has revived long-running questions about how much magma may be involved and how much warning the system would give before any major event.

Bradyseism: ground that breathes up and down

The defining behavior at Campi Flegrei is a phenomenon Italians call bradyseism, from Greek roots meaning “slow movement.” Rather than a single dome pushing up, the caldera floor rises and falls over months and years in alternating phases. Italy’s civil protection authorities describe the current unrest as comparable in scale to the crisis of 1982 to 1984, when uplift and earthquakes damaged buildings in the town of Pozzuoli badly enough to force evacuations from its historic center. Roughly half a million people live within the area exposed to the caldera’s activity, which is what turns a geological curiosity into a civil-protection problem.

The renewed rise began around 2005 and has continued in fits and starts ever since. The point of maximum deformation sits near Pozzuoli’s harbor, where the cumulative uplift has added up to well over a meter across the current episode. By the middle of 2025, the ground at the reference marker in the Rione Terra district had climbed roughly 148 centimeters since the uplift resumed, a figure monitored continuously by geophysical instruments ringing the bay.

Quakes that track the swelling

As the caldera has inflated, the earthquakes have grown more frequent and, at times, stronger. Researchers at Italy’s National Institute of Geophysics and Volcanology have quantified a direct link between the rate of ground uplift and the pace of seismic activity, so that faster rising tends to bring more tremors. The largest shocks recorded during the current phase have reached the mid-4 magnitude range, the strongest measured since instrumental monitoring began, and dense swarms of smaller quakes have repeatedly concentrated under the Pozzuoli and Agnano areas.

Most of these earthquakes are shallow and modest by global standards, but at Campi Flegrei even small quakes matter. Shallow, repeated shaking stresses old masonry, and the sheer number of events wears on residents and infrastructure. A detailed study of the caldera’s coupling between uplift and seismicity has helped scientists interpret how stress builds and releases in the rock above the pressure source.

The human dimension compounds the geological one. The towns atop the caldera are old and densely packed, and many buildings were never engineered to withstand repeated jolts. Authorities have run drills, reinforced structures and planned evacuation routes, but moving or protecting half a million people is a challenge with no easy solution, which is why continuous, precise monitoring is treated as essential rather than optional.

Magma or steam? The debate over what drives it

The central scientific question is what is actually pushing the ground up. The pressure source lies several kilometers below the surface, and analyses disagree on whether the deformation is driven mainly by molten rock rising toward the surface or by hot fluids and gases circulating in the hydrothermal system above a deeper magma body. A reassessment of the nature of the bradyseism underscores how much uncertainty remains about that source, which is the biggest obstacle to forecasting where the current episode is headed.

That distinction matters enormously for risk. A purely hydrothermal cause suggests the caldera is venting pressure without magma necessarily moving toward eruption, while clear evidence of rising magma would raise the stakes. Because the answer is not settled, authorities treat the unrest as a hazard to be monitored rather than a countdown, updating alert levels as the data change.

A supervolcano with a violent past

Campi Flegrei earns the “supervolcano” label from its history rather than its everyday behavior. Tens of thousands of years ago it produced one of the largest eruptions Europe has seen, blanketing a vast region in volcanic material, and it has erupted since then, most recently in 1538 when a new hill called Monte Nuovo rose in a matter of days. Nothing on that scale is forecast today, and the vast majority of the caldera’s activity remains slow uplift and minor quakes.

Still, the combination of a restless caldera and a metropolitan population keeps Campi Flegrei near the top of Europe’s volcanic-risk lists. History is part of the reason for caution: the 16th-century eruption showed the system can build a new vent quickly, and the caldera has cycled through periods of unrest before without erupting, which makes each new episode hard to interpret in isolation. The ongoing rise is a reminder that the ground beneath one of Italy’s most crowded coastlines is not fixed, and that understanding how the system breathes is the first line of defense for the people living on it.

This article was produced with AI assistance and reviewed by the Morning Overview editorial team.

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