Beneath the volcanic hills west of Naples, a vast underground system has been swelling, cracking, and grumbling for more than a decade, unsettling a region that is home to roughly half a million people. Unlike a single, distant peak, this restless ground is a caldera — the wide, collapsed footprint left behind by ancient eruptions — and it sits directly beneath towns, harbors, and city streets rather than in some remote wilderness. Scientists who track it describe a landscape that never fully sleeps, one where the ground itself visibly rises and falls over the course of years.
A Caldera Instead of a Cone
Campi Flegrei, Italian for the Phlegraean Fields, spans roughly 12 to 15 kilometers across, with much of its rim submerged beneath the waters of the Gulf of Pozzuoli. Rather than one dominant summit, the area is dotted with dozens of craters, hot springs, and fumaroles that vent steam and sulfurous gas into the air, a landscape so strange that ancient writers described it as an entrance to the underworld. The caldera formed after colossal eruptions emptied a shallow chamber of magma, causing the ground above it to collapse inward on itself, and the modern city of Pozzuoli sits almost entirely within that sunken rim, with greater Naples and its millions of additional residents spreading out just to the east.
Bradyseism: A Landscape That Rises and Falls
The defining feature of the caldera’s unrest is a slow, cyclical motion called bradyseism, in which the ground rises or sinks over years to decades as pressure builds or eases within the hydrothermal and magmatic systems underneath it. Ancient Rome left one of the clearest records of this movement: marble columns at a submerged market known as the Macellum, in Pozzuoli, still bear boreholes drilled by marine mollusks well above today’s ground level, proof that the site sank beneath the sea for centuries before rising again, as detailed in Wikipedia’s account of the Phlegraean Fields. A fresh episode of uplift began accelerating in the years after 2005, and parts of Pozzuoli’s ground have now risen more than a meter since an earlier uplift cycle in the mid-20th century.
The Eruption That Reshaped Prehistoric Europe
The caldera’s largest known outburst, called the Campanian Ignimbrite eruption, struck roughly 39,000 years ago and ranks among the most powerful volcanic events to hit Europe in the past 200,000 years, scattering ash across a wide swath of the continent and the eastern Mediterranean. Some researchers have linked the eruption’s climatic aftershocks to the decline of Neanderthal populations in the region, though that theory remains debated among archaeologists. A second, smaller caldera-forming eruption, known as the Neapolitan Yellow Tuff event, followed roughly 15,000 years ago. Since then, volcanic activity at the site has continued on a far smaller scale, most recently in 1538, when weeks of tremors and uplift culminated in the sudden birth of a new hill, Monte Nuovo, during the area’s last confirmed eruption.
Watching a System That Refuses to Go Quiet
Italy’s national institute for geophysics and volcanology, known as INGV, has logged thousands of small earthquakes beneath the caldera in recent years as part of an intensifying swarm that has included the strongest tremors recorded there in decades. Civil protection officials raised the official alert level in 2012 and have periodically debated whether current conditions justify raising it again, a step that would trigger expanded evacuation planning for the densely packed caldera floor. Researchers monitoring the unrest generally believe the earthquake swarms and rising ground point toward pressurized steam and fluids shifting through the hydrothermal system rather than magma pushing directly toward the surface, though distinguishing between the two processes in real time remains genuinely difficult. Even a hydrothermal event, rather than a true eruption, carries its own hazards, since sudden bursts of steam and boiling water have injured or killed visitors at other calderas around the world with comparatively little warning.
What a Bigger Eruption Could Look Like
Civil protection planners have modeled several eruption scenarios for Campi Flegrei, ranging from a relatively contained event similar to the 1538 birth of Monte Nuovo to a far larger explosive episode that would blanket the surrounding region in ash and force the evacuation of hundreds of thousands of residents before any eruption began. Because the caldera’s craters lie so close to dense neighborhoods, even a modest explosive eruption could disrupt the port of Naples, regional rail lines, and the area’s water and power networks well beyond the immediate blast zone. Officials have organized evacuation drills for towns within the caldera’s designated red zone, though logistics remain difficult given how many people would need to move along a limited road network in a short window.
Living Inside an Active Caldera
What sets Campi Flegrei apart from most other closely watched volcanic systems is simply how many people live on top of it. Pozzuoli alone counts tens of thousands of residents, while the wider caldera area and its adjoining Naples suburbs bring the population living within or immediately beside the crater rim to several hundred thousand people, woven among ports, rail lines, and dense apartment blocks. Evacuation planning for the region is complicated by narrow roads, aging infrastructure, and a population that dwarfs those living near almost any other actively monitored caldera on the planet. For now, the ground continues its slow rise, and seismometers keep recording the tremors that have turned this stretch of Italian coastline into one of the most intensively studied volcanic landscapes in the world.
This article was produced with the assistance of AI and reviewed by Morning Overview editors.
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