Anyone visiting Pozzuoli for the first time sees a perfectly ordinary Italian port city. Colorful houses, fishing boats, espresso along the waterfront, children playing in the piazza. Nothing suggests at first glance that one is standing on one of the most dangerous supervolcanoes on Earth.
Welcome to the Campi Flegrei, the Phlegraean Fields.
What exactly are the Phlegraean Fields?
The name comes from Greek: "phlegraios" means burning. And that is no exaggeration. The Phlegraean Fields are a vast caldera, a collapsed volcanic crater, stretching across roughly 150 square kilometers to the west of Naples. Pozzuoli lies right at its center. So do the districts of Bagnoli and Fuorigrotta, the Lago d'Averno, the hills of Bacoli, the ancient city of Cumae , and many other places that are today home to hundreds of thousands of people.
Unlike a classic volcano with a single cone, the caldera has no visible peak. The danger comes from below, from an enormous magma system deep beneath the earth that has kept the entire region in motion for thousands of years.
Bradyseism: when the city rises and falls
The most fascinating and at the same time most unsettling phenomenon of the Phlegraean Fields is bradyseism. The term also comes from Greek: "brady" means slow, "seismos" means movement. It refers to the gradual rising and sinking of the ground, caused by pressure from magma and hot gases deep beneath the surface.
In Pozzuoli, this phenomenon can be observed more clearly than anywhere else in the world. The most famous testament to it is the Macellum, the ancient Roman market in the old town. The columns of the Macellum bear the borings of marine mollusks several meters above their base. This means that these columns once stood under water. The ground had subsided so far that the sea flooded the building. Then it rose again. Today the columns stand in open air once more, and the traces left by the mollusks silently tell of centuries of geological movement.
Since 1950, the ground around Pozzuoli has risen by more than four meters in total, interrupted by recurring phases of subsidence. In the early 1980s, the situation became so serious that large parts of Pozzuoli's city center had to be evacuated. People returned, the earth settled, at least for a time.
The Solfatara: sulfur, steam, and an end-of-the-world atmosphere
Right in the heart of Pozzuoli lies the crater of the Solfatara, an active volcanic crater that has been steaming, bubbling, and reeking, in the best sense of the word, for centuries. Visiting the Solfatara means stepping into a lunar landscape: whitish-yellow ground from which hot sulfurous steam rises at dozens of points, gurgling mud pools, and a smell of rotten eggs that stubbornly clings to the nostrils.
Temperatures in the fumaroles, the steam vents, reach up to 160 degrees Celsius. The ground sounds hollow when tapped, and in some spots it is noticeably warm beneath the soles of one's shoes. It is a place that simultaneously fascinates and humbles, making it unmistakably clear: here, humanity is a guest, not at home.
The Solfatara has been known since antiquity. Roman writers described it, and early Christians believed they had found the entrance to the underworld here. Dante is said to have drawn inspiration from it.
The Lago d'Averno
Just a few kilometers from Pozzuoli lies the Lago d'Averno, a circular crater lake nearly two kilometers in diameter. The water is dark, the surroundings quiet, and the atmosphere has something otherworldly about it. In antiquity, the Romans believed this lake was the entrance to the underworld, and Virgil had Aeneas descend into it here.
On the shore of the lake, a monumental Roman bath complex ruinstill stands today, long mistakenly referred to as the Temple of Apollo. It is freely accessible and ranks among the most impressive and at the same time least known sights in the entire region.
The ancient Greek colony of Cumae
At the northwestern edge of the Phlegraean Fields lies Cumae, the oldest Greek colony on the Italian mainland, founded as early as the 8th century BC. From here, not only Greek ideas and goods spread across the region, but also the alphabet, which later became the foundation of Latin script. In Cumae, if you will, Europe's written history began.
Cumae is best known for the legendary Sibyl, a prophetess of Apollo whose oracle was hidden deep within the rock. The passage cut into the tufa stone, the Antro della Sibilla, remains to this day one of the most atmospheric sites in the entire region. And from the Acropolis , where temples to Apollo and Zeus once stood, the view stretches far across the Tyrrhenian Sea, over the same waters that the first settlers once crossed to arrive here.
Baiae: the sunken bathing paradise of the emperors
Just a few kilometers west of Pozzuoli lies Baiae, once the most coveted destination of the Roman elite. Emperors such as Hadrian, Nero, and Augustus spent their summers here in magnificent villas overlooking the sea, enormous thermal bath complexes and domed baths whose acoustics amplify even a whisper to this day. These were not sacred sites but places for relaxation, conversation, and the staging of power.
Part of the ancient city now lies underwater, submerged by the very same bradyseism that also shaped the Macellum in Pozzuoli. Where promenades and harbor facilities once stood, an Underwater Archaeological Park now takes visitors into the sunken world by glass-bottom boat or diving excursion. What has remained on dry land ranks among the most impressive and, at the same time, least known excavation sites in Italy.
A Harbor with World History
In antiquity, Pozzuoli itself was one of the most important harbors in the entire Roman Empire. Ships from Alexandria brought grain, from the Orient spices, from Sicily wine and oil. And it was at this very harbor that the Apostle Paul first set foot on Italian soil during his final journey to Rome, a moment recorded in the Acts of the Apostles that can still be traced today at a modest stone near the harbor.
The Campi Flegrei Today
Today, the area of the Campi Flegrei is a regional park with a unique blend of nature, archaeology, and everyday life. Residential buildings stand directly alongside ancient ruins. Hiking trails wind between sulfur springs. And from Pozzuoli, the ferry departs for Ischia, passing a gulf so beautiful that one almost forgets being on top of a volcano.