Phlegraean Fields: The Volcanic History of the Naples Caldera

I Campi Flegrei They are one of the most studied and fascinating volcanic areas on the planet. Beneath the municipalities of Pozzuoli, Bacoli, Quarto and part of Naples lies a boiler approximately 12 kilometers in diameter, largely submerged by the Gulf of Pozzuoli, capable of continental-scale eruptions.

The name comes from the Greek phlégraios, meaning "burning," "fiery": the Greeks who founded Cumae and Dicaearchia were well aware of the nature of this land that smokes, trembles, and rises. Today, the Phlegraean Fields are monitored in real time by the INGV, but they reveal a volcanic history spanning tens of thousands of years, made up of colossal explosions, ground upheavals, and cities built on craters.

In this guide, we reconstruct the volcanic history of the Phlegraean Fields, from the great prehistoric eruptions to the ongoing bradyseismic crisis, up to the seismic swarm of July 31, 2026.

What is the Campi Flegrei caldera?

To understand the Phlegraean Fields, one must forget the image of a conical volcano, like Vesuvius. Here the structure is a boiler: a large, flat, depressed area formed after enormous eruptions emptied the magma chamber below, causing the ground to sink hundreds of meters.

A volcano hidden beneath the city

The Phlegraean caldera extends from Monte di Procida to the Posillipo hill and continues under the sea of ​​the Gulf of Pozzuoli. Inside it there are at least 24 craters and volcanic cones, many of which today host neighborhoods, lakes and parks: the Astroni, Monte Nuovo, the Solfatara, Lake Averno, Lake Lucrino.

This is the most striking peculiarity: there are few active volcanic areas in the world with such a high population density. Hundreds of thousands of people live within the caldera's perimeter, and the area has been continuously inhabited for over three thousand years.

Caldera, not cone: the difference that matters

A volcanic cone erupts from a central point. A caldera, on the other hand, can erupt from different points, which change over timeEach Phlegraean crater has its own activity cycle, and the location of magma ascent can shift. This is why surveillance covers the entire area, not just a single volcanic edifice.

The eruptive history: 39.000 years of fire

The volcanic history of the Phlegraean Fields is marked by at least 70 eruptions over the last 39.000 years, with widely varying intensities. Two of these were among the largest ever to occur in Europe. Here's a timeline of the main events.

The Campanian Ignimbrite, 39.000 years ago

About 39.000 years ago the Phlegraean Fields produced the eruption of theBell Ignimbrite, one of the largest volcanic events in Europe in the last 200.000 years. Tens of cubic kilometers of magma were ejected, and the ash was found as far away as Greenland, where it was identified in ice cores.

The pyroclastic flows traveled for tens of kilometers, covering the entire Campanian plain. The event emptied the magma chamber and caused the ground to collapse, forming the caldera structure we see today.

Neapolitan Yellow Tuff, 15.000 years ago

A second colossal eruption, about 15.000 years ago, spewed out the Neapolitan Yellow TuffA deposit that today forms the tuff hills on which entire neighborhoods of Naples stand, from Posillipo to Vomero. The yellow Phlegraean tuffs were the city's building material for centuries.

It was on that occasion that the caldera reached its current configuration, with the collapse of the central part and the ingression of the sea into the Gulf of Pozzuoli.

The Main Pumice Eruption, 4.550 years ago

The last major explosive eruption dates back to about 4.550 years ago, when the Main Pumice event destroyed the Neolithic villages of the Phlegraean Fields, burying them under meters of pumice and ash. Excavations at Nisida and the Via Cinthia area have uncovered the remains of those communities.

Since then, the volcano has not produced eruptions of similar magnitude, but has remained active with minor events, hydromagmatic eruptions and intense hydrothermal activity.

Monte Nuovo, 1538: the last eruption

Between September 29th and October 6th 1538, in a few days, the earth opened up to the west of Pozzuoli: a crater and a 133 metre cone were born, the Monte Nuovo, which engulfed the village of Tripergole and its thermal baths. It was the last eruption of the Phlegraean Fields, and was observed and described by contemporaries with a detail that makes it unique in the history of volcanology.

Monte Nuovo is now a nature reserve, a perfectly preserved hill in the heart of the caldera, which can be visited on foot in about an hour.

Bradyseism, the breath of the caldera

The phenomenon that characterizes the Phlegraean Fields more than the eruptions is the bradyseism: the slow rising and falling of the ground, driven by the pressure of fluids and magma in the deep magma chamber. The name derives from the Greek bradýs, “slow”, and it is a slow but constant movement, measured in centimetres per year.

The columns of the Serapeum, the proof written in marble

Il Serapeum of Pozzuoli It is the monument that made bradyseism famous. Its marble columns display holes from lithodomes, mollusks that live only in seawater, up to seven meters above the current level: evidence that the ground sank beneath the sea in Roman times and then rose again several times in the following centuries.

Since 1950 the port of Pozzuoli has risen by a total of approximately 4,5 meters, in three major phases: 1969-72, 1982-84 and the crisis that began in 2005 and is still ongoing.

The crises of 1969-72 and 1982-84

During the 1982-84 crisis, the ground rose almost two meters in two years, with thousands of earthquakes. The historic center of Pozzuoli, the Rione Terra, was evacuated: approximately 16.000 people left the area. It was the most severe bradyseismic crisis ever observed with modern instruments.

Since then, the INGV Vesuvius Observatory has monitored the area with a network of instruments that is among the densest in the world.

The ongoing crisis and the swarm of July 31, 2026

Since 2005, the caldera has begun to rise again, with an acceleration in recent years. The alert level is yellow, and INGV bulletins record hundreds of earthquakes per month. In June 2026, 377 events were detected, with a maximum magnitude of 3.6.

On the evening of July 31, 2026, the area was shaken by the most intense seismic swarm in recent yearsThe main shock, a magnitude 4.7 earthquake with a hypocenter about 3 kilometers between Pozzuoli and Quarto, caused ten minor injuries, collapsed cornices, and disruption to traffic on the Cumana and Circumflegrea lines, which were suspended for inspection. As we explain in the article on the damage from the July 31 earthquake, the situation was managed by securing buildings and conducting inspections in the red-zone municipalities.

How the volcano works

The engine of the Phlegraean Fields is deep down: a magma chamber Located 6 to 8 kilometers below the surface, fed by a deeper reservoir. The rising magma cools and releases gases, and it is precisely these high-pressure fluids that lift the ground and generate earthquakes.

The magma chamber and the gases

When gases released from magma (primarily carbon dioxide and water vapor) accumulate in fracture systems, pressure increases: the ground swells like a balloon, and rocks break, producing seismic swarms. This is the dominant mechanism today, according to the most recent studies by the INGV.

This explains why the current activity is mostly seismic and uplift, without scientific evidence of magma rising to the surface enough to herald an eruption in the short term.

Fumaroles, Solfatara and Pisciarelli

The vitality of the system is visible on the surface: dozens of fumaroles They emit steam and gas at temperatures exceeding 90 degrees. The best-known sites are the Solfatara, with its fumaroles and boiling mud pools (closed to the public since 2017), and the area of Pisciarelli, between Pozzuoli and Monte Nuovo, where hydrothermal activity is among the most dynamic in Europe.

For those who want to see these landscapes up close, Monte Nuovo, the Astroni, and Lake Averno are the best spots: find routes, trails, and guided tours in our guide to excursions to Vesuvius and the Phlegraean Fields.

A volcano under special surveillance

The Phlegraean Fields are among the most monitored volcanoes on the planet.Vesuvius Observatory, the Neapolitan section of the INGV, manages a dense network of instruments distributed throughout the caldera, with data transmitted in real time to the operations rooms.

The monitoring network

Hundreds of sensors measure every parameter: seismographs for earthquakes, GPS and inclinometers for soil deformation, tide gauges For movements relative to sea level, instruments for soil and fumarole gases. The INGV's weekly and monthly bulletins publicly update all data.

Alongside science there is civil protection: the evacuation plan for the red zone involves approximately 500.000 residents in 18 municipalities, with an organization that provides for the transfer within defined timeframes to the twinned regions.

FAQ about the Phlegraean Fields

The Phlegraean Fields aren't just a risk to be managed: they're a unique landscape, where the volcano's millennia-old history intertwines with that of a city that has always coexisted with it. Understanding its volcanic history is the first step to experiencing it with awareness, amidst fumaroles, volcanic lakes, and hills born of fire.