A restless supervolcano near Naples has been bulging and rattling for months

West of Naples, Italy, an enormous volcanic caldera has been quietly rising and falling for years, unsettling the ground beneath hundreds of thousands of residents without producing an actual eruption. Known as Campi Flegrei, or the Phlegraean Fields, this restless volcanic system differs from the classic image of a single conical mountain like nearby Vesuvius. Instead, it is a wide depression formed by past eruptions so massive they helped shape the regional landscape, and its current unrest has kept scientists and civil authorities watching closely for any sign that the slow deformation might be building toward something more dangerous.

A Caldera That Rises and Falls Instead of Erupting

Campi Flegrei is a large volcanic caldera, a broad, bowl-shaped depression that formed when previous massive eruptions emptied vast underground chambers of magma, causing the ground above to collapse inward. Unlike a single-peaked stratovolcano, the caldera spans a wide area that includes densely populated neighborhoods, coastal towns, and part of the city of Naples itself, making any significant activity there a matter of direct concern for a large regional population.

The ground movement scientists have tracked in recent years is a phenomenon known as bradyseism, a slow rising and sinking of the Earth’s surface driven by the movement of magma, gases, and fluids in the volcanic system beneath it. This uplift and subsidence has occurred in cycles over centuries, sometimes rising for years before reversing into a period of sinking, without necessarily indicating that an eruption is imminent.

A History of Catastrophic Eruptions

Campi Flegrei’s current status as one of the few volcanic systems classed as a supervolcano rests on its ancient eruptive history rather than on anything happening today. Tens of thousands of years ago, the system produced one of the largest eruptions in European history, an explosion powerful enough to send ash across a huge stretch of the surrounding region and to reshape the local landscape into the broad depression that later became Campi Flegrei itself. A second, smaller but still enormous eruption thousands of years later helped further define the caldera’s modern boundaries.

Since those ancient super-eruptions, the caldera has produced numerous smaller eruptions, the most recent occurring centuries ago rather than in the current era. That gap has allowed the region around the caldera to become densely settled over time, which is precisely what makes the system’s current unrest such a significant concern for modern civil planners in a way it never was for the ancient communities that lived through the caldera’s last active period.

Decades of Bradyseism West of Naples

The town of Pozzuoli, sitting at the heart of the caldera, has experienced some of the most dramatic effects of this ground movement, including past episodes of uplift significant enough to damage buildings and force temporary evacuations of some neighborhoods. Ancient Roman ruins in the area, including columns at a well-known marketplace site, carry visible marks left behind by centuries of the ground rising and falling relative to sea level.

That long historical record gives scientists a valuable baseline for understanding the current unrest, since Campi Flegrei has gone through comparable episodes of uplift and seismic activity in past centuries without necessarily culminating in a major eruption. Even so, each new episode of accelerated movement prompts renewed monitoring, since the underlying magma system remains capable of producing eruptions ranging from relatively minor to historically catastrophic.

Why the Ground Keeps Moving Under Pozzuoli

Scientists generally attribute the ongoing bradyseism at Campi Flegrei to a combination of magma movement deep underground and the behavior of a hydrothermal system, superheated water and gases trapped within the volcanic rock. Pressure changes in that system can push the surface upward, while the release of pressure through gas emissions or fluid movement can allow the ground to sink back down.

This mechanism explains why the caldera can experience years of uplift accompanied by swarms of small earthquakes without the kind of magma reaching the surface that would be needed for an actual eruption. It does not, however, fully rule out the possibility that sustained unrest could eventually be followed by volcanic activity, which is why continuous monitoring remains a priority for Italian volcanologists and civil protection authorities.

Distinguishing between unrest that will fade on its own and unrest that signals magma actually pushing toward the surface is one of the central challenges in volcanology generally, not just at Campi Flegrei. Scientists look for specific changes in the pattern of earthquakes, shifts in the ratio of different gases escaping from vents, and changes in the rate of ground uplift as possible warning signs that a system is behaving differently than it has during past episodes of bradyseism that ultimately did not lead to an eruption.

Because Campi Flegrei has produced false alarms in the past, cycles of uplift and seismicity that eventually subsided without an eruption, scientists are cautious about treating any single episode of unrest as decisive proof that conditions have fundamentally changed. That caution reflects an effort to avoid both missing genuine warning signs and triggering unnecessary evacuations based on ambiguous data.

How Scientists Monitor a Restless Supervolcano

Researchers track Campi Flegrei using a dense network of seismometers, GPS stations, and gas-monitoring equipment positioned throughout the caldera, measuring even small changes in ground elevation, seismic activity, and the chemical composition of gases escaping from vents in the area. This continuous data stream allows scientists to detect changes in the system’s behavior far more quickly than would have been possible in past centuries.

Because Campi Flegrei sits beneath a heavily populated area, that monitoring feeds directly into regional emergency planning, including evacuation strategies designed for the communities living closest to the caldera. Authorities have organized the surrounding zone into risk areas used to guide evacuation planning, an approach borrowed in part from the long-standing emergency planning built around Vesuvius on the opposite side of Naples, itself famous for burying the Roman cities of Pompeii and Herculaneum in antiquity. The current period of unrest has kept that planning especially active, even though scientists continue to emphasize that ground deformation and swarms of small earthquakes, on their own, do not necessarily mean an eruption is close at hand.

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

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