The ground under half a million people near Naples keeps rising

West of Naples, an entire district of homes, roads, and ancient Roman ruins sits on ground that rises and falls over years and decades, driven by forces churning beneath one of the largest volcanic calderas on the planet. Hundreds of thousands of people live directly on top of it, and the ground has been moving again.

Campi Flegrei, a caldera hiding in plain sight

Unlike a volcano with an obvious peak, Campi Flegrei, also called the Phlegraean Fields, is a sunken, largely flat depression that spans the coastal area immediately west of Naples, including the town of Pozzuoli and part of the Bay of Naples itself. The caldera formed through two enormous eruptions in the distant past, one roughly 39,000 years ago and another about 15,000 years ago, each collapse leaving behind the wide, low-lying basin that residential neighborhoods, industrial buildings, and stretches of the local rail network now sit on top of.

Bradyseism: ground that breathes

The defining feature of the area is bradyseism, a slow, uneven rise and fall of the caldera floor driven by the movement of magmatic and hydrothermal fluids deep underground. During active phases, parts of the ground can lift by tens of centimeters in a single year, a pace fast enough to crack roads, tilt buildings, and force the closure of some structures near the port of Pozzuoli. The phenomenon has been documented for centuries; Roman-era columns at the nearby Macellum market, sometimes called the Temple of Serapis, still show marine boring holes partway up their shafts, physical evidence that the site once sank low enough to be submerged before rising again over subsequent centuries.

Hundreds of thousands of residents inside the risk zone

What sets Campi Flegrei apart from most other high-hazard volcanic systems is not the size of a potential eruption but the density of the population living directly inside the caldera. Pozzuoli and the surrounding districts hold several hundred thousand residents packed into an urban and suburban landscape that offers few easy evacuation routes, since much of the area is hemmed in by the sea on one side and the city of Naples on the other. Naples itself, with its own metropolitan population numbering in the millions, sits close enough to the caldera’s eastern edge that any large-scale evacuation plan has to account for both communities at once, not just the smaller towns closest to the caldera’s center. That combination, a restless caldera and a crowded evacuation zone, has made Campi Flegrei one of the volcanic sites monitoring agencies watch most closely anywhere in Europe.

Ongoing unrest and low-level seismicity

The caldera has remained under an elevated monitoring status for years, with ongoing bradyseism accompanied by hundreds of small earthquakes recorded most months and increased emissions from fumarole fields at Solfatara and Pisciarelli, two sites within the caldera where volcanic gases vent to the surface, patterns described in the overview of Campi Flegrei maintained on Wikipedia. None of that activity indicates an eruption is underway or imminent; earthquake swarms and gas emissions of this kind have occurred in cycles at Campi Flegrei for decades without triggering renewed volcanic activity. Instead, scientists treat the pattern as a sign that the underlying hydrothermal and magmatic system remains active and needs continuous observation rather than a fixed warning of collapse. Most of the recorded earthquakes are too small to be felt at the surface, registering as background seismicity that only shows up clearly on instruments run by Italy’s national geophysics institute.

A history of false alarms and real caution

Campi Flegrei experienced a major bradyseismic crisis between 1982 and 1984 that forced the evacuation of tens of thousands of Pozzuoli residents after the ground rose by nearly two meters over roughly two years, damaging buildings across the town center. That episode ended without an eruption, and the ground later subsided again, though not all the way back to its starting point. The 1980s crisis remains the primary case study Italian civil protection authorities use when planning for a future scenario at the site, since it demonstrated both how quickly conditions can escalate and how a bradyseismic episode can also resolve without a volcanic event. Smaller uplift episodes have followed in the decades since, each one prompting renewed public attention and additional rounds of building inspections in the most affected neighborhoods.

Because the caldera’s behavior does not follow a simple, predictable timeline, Italian volcanologists rely on continuous ground-deformation measurements, gas composition sampling, and seismic monitoring networks to track whether current unrest is following a pattern similar to past episodes or diverging into new territory. That data feeds directly into the alert level assigned to the area, which governs evacuation planning for the dense population living on top of one of the few calderas in the world capable of a large-scale eruption directly beneath a modern metropolitan area. Researchers studying the caldera’s deep structure have also used the accumulated data to refine estimates of how much magma remains stored beneath the surface, work that shapes how seriously each new uplift episode is treated by civil protection planners.

This article was created with the assistance of AI and reviewed by an editor.

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