Campi Flegrei, or the “Phlegraean Fields” (from the Greek phlegos, meaning “burning”), is not a typical volcano. You won’t find a single towering cone like its neighbor, Vesuvius. Instead, it is a massive, depressed volcanic area—a caldera—stretching 13 kilometers (8 miles) across. It lies mostly underwater and under the bustling suburbs of Naples, home to hundreds of thousands of people.

It is often classified as a supervolcano, a term that evokes doomsday scenarios. While its history is indeed violent, Campi Flegrei is also a land of incredible beauty, rich history, and unique geological phenomena that have fascinated humans for millennia. From the ancient Romans who built their holiday villas here to modern geologists monitoring its every breath, this fiery landscape is one of the most intriguing places on Earth.

A History of Violence: The Super-Eruptions

The landscape we see today was shaped by two cataclysmic events that dwarf almost any historical eruption.

The Campanian Ignimbrite (39,000 years ago): This was the “big one.” It ejected an estimated 200–500 cubic kilometers of magma. The ash cloud blanketed a vast area of Eastern Europe and Russia. Some scientists believe the volcanic winter caused by this eruption may have contributed to the final extinction of the Neanderthals. It led to the collapse of the ground, creating the caldera’s initial shape.
The Neapolitan Yellow Tuff (15,000 years ago): A second massive eruption produced the characteristic yellow stone (tuff) that Napolitans have used to build their city for centuries. This event further defined the caldera structure.
Monte Nuovo (1538 AD): The volcano’s only eruption in recorded history was a mere hiccup compared to its past. Over the course of a week, a brand new mountain (“Monte Nuovo”) grew from the ground, burying the medieval village of Tripergole. It was the frantic finale to a period of intense ground uplift.

Bradyseism: The Breathing Ground

The most unique and unsettling feature of Campi Flegrei is bradyseism. The ground here does not just shake; it rises and falls slowly over time, like the chest of a sleeping giant. This is caused by the movement of fluids and gases in the hydrothermal system beneath the crust, and possibly by the accumulation of magma.

The Temple of Serapis: A Geological Gauge

In the port of Pozzuoli stands the ancient Roman Macellum (market), misidentified early on as the Temple of Serapis. It is famous not for its architecture, but for its columns.

The Evidence: Dark bands of holes bored by marine mollusks (Lithophaga) can be seen halfway up the columns.
The Story: These holes prove that the temple was once submerged underwater due to the ground sinking (subsidence), only to be pushed back up above sea level later (uplift). It is a stone record of centuries of bradyseism.

The Crisis of the 1980s

In the early 1970s and again in 1982-1984, the ground rose rapidly—nearly 3.5 meters in total. The port of Pozzuoli became too shallow for ships. Walls cracked. Fear of an imminent eruption led to the mandatory evacuation of 40,000 people from the historic center of Pozzuoli (Rione Terra). The eruption never came, but the town was changed forever. Since 2005, the ground has been rising again, accompanied by frequent seismic swarms that keep the local population on high alert.

Solfatara: The Gateway to Hell

The Solfatara crater is the most active part of the caldera today. For centuries, it was a major tourist attraction.

The Landscape: It looks like a scene from another planet—a stark, white landscape of sulfurous rocks and hissing fumaroles releasing steam at 160°C.
The Myth: The ancients believed this was the entrance to the Underworld. It was here (or at nearby Lake Avernus) that Aeneas descended into Hades in Virgil’s Aeneid.
Current Status: Historically open to visitors who could walk among the fumaroles, the crater has been closed to the public since 2017 following a tragic accident where a family fell into a chasm. It remains closed as of 2025, though its steam plumes can still be seen from the surrounding perimeter.

Baiae: The Sunken Las Vegas of Rome

If you want to see the power of bradyseism, look underwater. Baiae (modern Baia) was the luxury resort town for the Roman elite. Julius Caesar, Nero, and Hadrian all had villas here. It was a place of hedonism, thermal baths, and opulence.

As the ground slowly sank over the centuries, the lower part of the city vanished beneath the waves. Today, it is the Underwater Archaeological Park of Baiae.

Diving History: Scuba divers and snorkelers can swim over mosaics that once decorated Roman dining rooms. You can see statues still standing on their pedestals, now colonized by barnacles and fish.
Glass-Bottom Boats: For those who don’t dive, glass-bottom boats offer a glimpse into this submerged Atlantis.

Other Key Sites

Lake Avernus: A volcanic crater lake that the Romans believed was the mouth of Hell because birds flying over it would drop dead from toxic gases (Avernus comes from the Greek aornos, “without birds”). Today, it is a peaceful nature reserve.
Cuma (Cumae): The first Greek colony on the Italian mainland. It is home to the Cave of the Sibyl, a mysterious trapezoidal tunnel where the famous oracle prophesied the future.
Piscina Mirabilis: A colossal Roman cistern carved into the rock, looking more like an underground cathedral than a water tank. It supplied water to the Roman fleet stationed at Misenum.

Practical Information

Getting There: The Campi Flegrei are easily accessible from Naples. take the Cumana or Circumflegrea railway lines from Montesanto station in Naples. The trip takes about 20-40 minutes depending on your destination (Pozzuoli, Fusaro, Torregaveta).
Visiting:
- Flavian Amphitheater: Located in Pozzuoli, it is the third-largest Roman amphitheater in Italy. Its underground chambers are incredibly well-preserved and open to visitors.
- Rione Terra: The evacuated old town of Pozzuoli sits on a tuff cliff. After decades of abandonment, parts of it have been restored. Beneath it lies a complete Roman city streetscape that can be visited on guided tours.
- Baiae Castle: Houses the Archaeological Museum of the Phlegraean Fields, with artifacts recovered from the sea.
Safety: While the volcano is monitored 24/7 by the INGV (National Institute of Geophysics and Volcanology), it is an active seismic zone. Small earthquakes are common during uplift phases. Always check current alert levels before visiting active zones.