If Campi Flegrei erupted, the consequences would depend enormously on the size of the eruption, ranging from a localized explosion affecting a few kilometers to a catastrophe that reshapes southern Italy and disrupts climate across the Northern Hemisphere. The caldera sits beneath one of the most densely populated volcanic zones on Earth, with over half a million people living inside the designated high-risk “red zone” around its eight-mile-wide crater. Even a moderate eruption would be devastating. A large one would rank among the worst natural disasters in recorded history.
Not All Eruptions Are Equal
Campi Flegrei has produced eruptions across a wide spectrum of power. Scientists use the Volcanic Explosivity Index (VEI), a scale from 0 to 8, to classify them. The caldera’s history includes everything from small phreatic blasts (steam-driven explosions with no fresh magma) to some of the largest eruptions Europe has seen in the last 200,000 years. Three stand out as benchmarks for what this system can do.
The most recent large eruption, Monte Nuovo in 1538, was tiny by Campi Flegrei standards. It built a small cone and would barely register against the caldera’s major events. The Neapolitan Yellow Tuff eruption roughly 15,000 years ago was a VEI 6 event, powerful enough to form the caldera itself and send deadly flows up to 40 kilometers from the vent. The Campanian Ignimbrite eruption around 40,000 years ago was a VEI 7, the largest eruption in Europe in the past 200,000 years, with flows reaching an astonishing 150 kilometers from the source.
The most likely scenario, based on the caldera’s eruptive frequency, is something smaller: a VEI 3 or 4 eruption, comparable to many of the dozens of eruptions Campi Flegrei produced between its major caldera-forming events. Researchers have identified at least 29 eruptions spanning a 50,000-year window before the Campanian Ignimbrite, and at least nine eruptions in the 16,000 years leading up to it. Most of those were far smaller than the headline-grabbing supervolcanic events.
Pyroclastic Flows: The Primary Killer
The most immediate and lethal threat from a Campi Flegrei eruption is pyroclastic flows, fast-moving avalanches of superheated gas, ash, and rock fragments that hug the ground and incinerate everything in their path. These flows travel too fast to outrun and are hot enough to kill on contact.
In a VEI 5 eruption (large but not caldera-forming), modeling shows pyroclastic flows would push beyond 25 kilometers from the vent, easily overtopping the caldera rim and spreading into surrounding communities. A VEI 6 event would send flows more than 30 kilometers out. The Neapolitan Yellow Tuff eruption produced flows with initial velocities around 30 meters per second, accelerating to roughly 63 meters per second (about 140 mph) as they expanded outward.
A VEI 7 repeat of the Campanian Ignimbrite would be in a different category entirely. Those pyroclastic flows launched at speeds exceeding 80 meters per second (nearly 180 mph) and traveled far enough to reach the Roccamonfina volcano to the north, penetrate valleys deep in the Apennine Mountains, cross the sea, and surge over the Sorrento Peninsula to the south. Nothing survived in that footprint.
What Would Happen to Naples
Naples, a city of roughly 3 million people in its metropolitan area, sits directly east of the caldera. In any eruption of VEI 5 or above, pyroclastic flows would reach the city. Probabilistic hazard models confirm that flows from VEI 6 and VEI 7 scenarios would affect “a large part of the Campanian Plain to the north and the city of Naples to the east.” For a VEI 7 event, the destruction zone would extend well beyond the city in every direction.
Even in smaller eruptions, Naples would face heavy ashfall, which collapses roofs, contaminates water supplies, grounds aircraft, and makes roads impassable. Fine volcanic ash irritates lungs and can cause respiratory failure in vulnerable people. The weight of accumulated ash on flat-roofed Mediterranean buildings would be a serious structural hazard.
The Italian government has drawn a “red zone” around the caldera where evacuation would be mandatory before an eruption. Over half a million people live in this zone. But the practical challenge of moving that many people, through congested roads on a peninsula with limited escape routes, is one of the most difficult civil protection problems in the world. Italian authorities have developed evacuation plans assigning different municipalities to specific regions across Italy, but the plans assume days of warning time that a rapidly escalating eruption might not provide.
Ashfall and Climate Effects
Beyond the immediate kill zone, a large eruption would blanket hundreds or thousands of square kilometers in volcanic ash. During the Campanian Ignimbrite eruption, ash deposits spread across the entire Mediterranean region, forming a recognizable layer in sediments from the eastern Mediterranean to central Europe. A similar eruption today would shut down European airspace for weeks or longer, disrupt agriculture across southern Europe, and contaminate water systems far from the eruption site.
A VEI 7 eruption would also inject massive quantities of sulfur compounds into the stratosphere. These particles reflect sunlight back into space, cooling the planet. The Campanian Ignimbrite eruption is thought to have contributed to a period of cooling in the Northern Hemisphere, compounding the already harsh conditions of the last ice age. A modern equivalent could lower global temperatures by 1 to 2 degrees Celsius for a year or more, shortening growing seasons and reducing crop yields worldwide. The 1815 eruption of Tambora (also VEI 7) caused the “Year Without a Summer” in 1816, with crop failures across Europe and North America. A Campi Flegrei VEI 7 would produce comparable or worse climate disruption.
A smaller eruption, VEI 4 or 5, would still produce significant local ashfall and air quality problems but would not meaningfully alter global climate.
Tsunami Risk
Part of the Campi Flegrei caldera lies beneath the Gulf of Pozzuoli in the Tyrrhenian Sea. An eruption centered on or near the underwater portion of the caldera could displace large volumes of seawater, generating tsunamis along the coastlines of the Bay of Naples. Pyroclastic flows entering the sea at high speed can also trigger waves. The coastal towns ringing the bay, including parts of Naples itself, would face this compounding hazard on top of everything else. In a VEI 6 or 7 scenario, tsunami waves could propagate across the Tyrrhenian Sea and affect coastlines in Sardinia, Corsica, and North Africa, though with reduced height at those distances.
Economic and Humanitarian Fallout
Southern Italy’s Campania region has a GDP comparable to a mid-sized European country. A major eruption would effectively erase the economic output of the region for years. The port of Naples is one of Italy’s busiest. The area’s agriculture, tourism, and manufacturing would halt. Insurance losses would be in the hundreds of billions of euros for a large event, and much of the property in the high-risk zone is uninsured against volcanic damage.
The humanitarian crisis would extend well beyond the death toll. Displacing half a million to several million people, depending on eruption size, would strain Italy’s capacity and likely require international assistance on a scale not seen for a natural disaster in a developed nation. Resettlement, economic recovery, and rebuilding would take a generation.
How Much Warning Would There Be
Campi Flegrei is one of the most intensively monitored volcanoes on Earth. Italy’s National Institute of Geophysics and Volcanology operates networks of seismometers, GPS stations, gas sensors, and ground deformation monitors across the caldera. The ground has been rising since 2005, with increasing earthquake swarms, which is why the Italian government raised the alert level to “yellow” (elevated attention) in recent years.
Historically, large caldera eruptions are preceded by weeks to months of escalating activity: stronger earthquakes, faster ground uplift, changes in gas composition as magma moves closer to the surface. Scientists would almost certainly detect an approaching eruption days to weeks in advance. The critical uncertainty is distinguishing between unrest that fizzles out (as it did in the 1980s, when the ground rose nearly two meters without an eruption) and unrest that culminates in an actual eruption. That ambiguity makes the decision to evacuate half a million people politically and logistically agonizing, because a false alarm would cost billions and erode public trust for the next crisis.