Exploring the Campi Flegrei & Underground Naples: Volcanic Craters, Ancient Tunnels, and Hidden Ruins

What Makes This Corner of Naples So Extraordinary

Most visitors to Naples spend their days eating pizza on Spaccanapoli, climbing Vesuvius, and wandering Pompeii. All of that is worth doing. But west of the city, stretching toward Pozzuoli and the Tyrrhenian coast, lies one of the most geologically and historically layered landscapes in all of Europe — and almost nobody goes there. The Campi Flegrei, which translates loosely as “Burning Fields,” is a vast active supervolcanic system that has been bubbling, steaming, and occasionally exploding for tens of thousands of years. Beneath and around it, the ancient Greeks and Romans built cities, temples, market halls, and harbors. Some of those structures now sit underwater. Others are half-buried by centuries of volcanic sediment. Some are simply forgotten. Combine this with the extraordinary network of tunnels, cisterns, and catacombs that runs beneath Naples itself, and you have a destination that rewards curiosity in ways that few places in Europe can match. This guide covers the volcanic craters, sunken ruins, ancient tunnels, and underground Naples in practical detail, so you can explore it all without the crowds that swamp Pompeii.

The Campi Flegrei: Understanding the Supervolcano Beneath Your Feet

Before you set foot in Pozzuoli or descend into any tunnel, it helps to understand what you’re standing on. The Campi Flegrei is a caldera — a collapsed volcanic structure roughly 13 kilometers across — formed about 39,000 years ago by one of the largest eruptions in European prehistoric history. Unlike Vesuvius, which is a single cone you can see from anywhere in the bay, Campi Flegrei has no dramatic peak. Instead, its volcanic energy is distributed across dozens of craters, fumarolic fields, hot springs, and zones of bradyseism — a slow vertical movement of the earth’s surface caused by magma shifting underground.

Bradyseism is one of the stranger phenomena you’ll encounter here. The ground in parts of Pozzuoli has risen and fallen by several meters over recorded history — Roman columns in the Temple of Serapis bear the biological trace of this movement in the form of boreholes made by mollusks, showing that the columns were submerged, then re-emerged, over centuries. Scientists monitor the Campi Flegrei constantly. It is classified as a supervolcano, and its alert level has been raised in recent years as seismic activity increases. This is not a reason to avoid it — the area is inhabited by hundreds of thousands of people, and the risk of an imminent eruption is considered low — but it gives the landscape an undeniable psychological weight. You are not visiting a museum. The ground is alive.

The Campi Flegrei was also the setting for the mythological entrance to the underworld. Lake Avernus, a volcanic crater lake just inland from Baia, was believed by the Romans to be the mouth of Hades. Virgil set scenes from the Aeneid here. The noxious gases that once rose from the lake’s surface — now largely dissipated — killed birds flying overhead, feeding the legend. Even today, standing at its dark, still water surrounded by dense forest, it is not difficult to see why the Romans believed what they believed.

Pozzuoli: The Living City on a Restless Caldera

Pozzuoli is the urban heart of the Campi Flegrei and the place where most visits to this region naturally begin. It is a proper working city — about 80,000 people, a busy fishing port, the smell of frying seafood drifting off the waterfront, teenagers on scooters weaving between parked cars — and it happens to sit directly on top of one of the most volatile volcanic systems in the world. The juxtaposition is very Neapolitan. Life happens here regardless of what the earth is doing.

The old town climbs steeply above the port, and its centerpiece is the Anfiteatro Flavio, the third-largest Roman amphitheater ever built, after the Colosseum and the one in Capua. It seated roughly 40,000 spectators. What makes it exceptional is the state of its underground infrastructure — the hypogeum, the network of tunnels and cages beneath the arena floor where animals were kept before fights, is remarkably well-preserved and fully accessible to visitors. Walking through these corridors, which still have their original hoisting mechanisms and trapdoor shafts, gives you a visceral sense of the machinery of Roman spectacle that the Colosseum’s more famous but more crowded underground cannot always provide.

Down near the harbor, the Macellum of Pozzuoli — often misidentified as the Temple of Serapis, a name that stuck despite being historically inaccurate — is a Roman market hall dating to the 1st century AD. The three standing marble columns are famous in geological circles because their middle sections are riddled with holes made by date mussels (Lithophaga lithophaga), which only live in saltwater. This means the columns were submerged at some point after the market was built, then lifted back above sea level by bradyseism. It is, in a strange way, one of the most elegant geological records you will ever see preserved in stone.

The waterfront promenade in Pozzuoli is excellent for lunch. Restaurants here specialize in whatever came off the boats that morning — grilled orata, spaghetti alle vongole, fritto misto — and prices are considerably lower than in central Naples. Avoid anywhere with photographs on the menu or a tout at the door, and you will eat well.

Solfatara and Astroni: Walking Inside a Volcano

A short walk or bus ride from central Pozzuoli, the Solfatara crater is one of the most viscerally strange places you can visit in Europe. It is a dormant volcanic crater — dormant, not extinct — where the earth exhales constantly through fumaroles, vents that release sulfurous steam at temperatures reaching 160°C. The ground in places is warm beneath your feet. The smell of sulfur is persistent and unmistakable. The mud pools bubble. The steam columns hiss. It looks, without exaggeration, like the surface of another planet.

Solfatara was used by the Romans as a source of sulfur and for its supposedly therapeutic mud. It has been a tourist attraction — in the loosest sense of the word — since at least the 18th century, when Grand Tour visitors came to marvel at it. The crater is about 770 meters across and you can walk a circular trail around and across it. There are moments when the ground produces a hollow, booming sound if you stamp your feet — the result of cavities just beneath the surface. The site was closed for several years following a tragic accident in 2017, but it has since reopened with improved safety measures and a revised visitor route.

A few kilometers inland, the Riserva Naturale Cratere degli Astroni is a completely different experience. It is a volcanic crater that has been a nature reserve since the 1990s, managed by the World Wildlife Fund. Inside the caldera, which last erupted roughly 3,800 years ago, there is a dense temperate forest surrounding a series of small lakes. It feels utterly unlike the surrounding suburban landscape. Herons, raptors, and dozens of bird species nest here. Guided walks are available on weekends. The contrast with Solfatara — one crater raw and smoking, the other green and lushly overgrown — illustrates how varied the Campi Flegrei’s character can be across a very short distance.

Baia: The Sunken City and Its Underwater Archaeological Park

Of all the places in the Campi Flegrei, Baia is the one that most stops people in their tracks when they learn it exists. During the Roman Republic and Empire, Baia was the most fashionable resort town in the ancient world. Julius Caesar, Cicero, Augustus, Nero — they all had villas here. The thermal waters, the mild climate, the extraordinary views across the bay made it the kind of place where power went to relax, conduct politics informally, and build opulent architecture. Nero famously had his mother Agrippina murdered nearby.

Then the bradyseism came. Over centuries, the ground sank. The sea crept in. By the medieval period, most of ancient Baia was underwater. Today, the Parco Archeologico Sommerso di Baia — the Underwater Archaeological Park — protects the submerged ruins of the city, which lie in roughly 2 to 15 meters of water in the bay just offshore. You can visit by glass-bottom boat, which gives a surprisingly clear view of mosaic floors, columns, statues (reproductions — the originals are in the museum), and the outlines of entire building complexes below the surface. Experienced divers can arrange guided underwater tours through licensed operators in Pozzuoli.

On land, the Castello di Baia houses the Museo Archeologico dei Campi Flegrei, which contains some of the finest Roman sculpture recovered from this region, including a stunning cast of the Sacello degli Augustali (a room reconstructed from its original sunken location) and portrait busts of emperors and imperial family members. The collections here are genuinely world-class and almost entirely overlooked by international tourism. On a weekday, you may find yourself essentially alone with Roman marble.

Above the castle, the Parco Archeologico di Baia preserves the remains of an enormous imperial-era thermal complex, spread across terraced hillside gardens. The thermal baths of Baia were among the most elaborate ever built, fed by natural geothermal heat. Walking through the domed halls and arched corridors — some still partially intact — with the bay glittering below is one of the better experiences in this entire region.

Cumae and the Sibyl’s Cave: Where Myth Meets Ancient Stone

Further west along the coast, the Parco Archeologico di Cuma protects the ruins of Cumae, which was the first Greek colony on the Italian mainland, founded around 740 BC. Its position — a dramatic volcanic promontory overlooking the sea, with Lake Avernus and the Campi Flegrei at its back — made it one of the most strategically and symbolically powerful sites in early Italy. The Romans associated it deeply with prophecy and the boundary between the living and the dead.

The site’s most extraordinary feature is the Antro della Sibilla — the Cave of the Cumean Sibyl. This is a long, precisely cut trapezoidal tunnel carved into the volcanic tufa rock of the hill, stretching roughly 130 meters into the hillside before opening into a vaulted chamber where the Sibyl was said to prophesy. Virgil describes Aeneas descending here to consult the Sibyl before his journey to the underworld. The tunnel, with its series of lateral light shafts cutting diagonal beams across the corridor, is architecturally and atmospherically unlike almost anything else in Italy. It was rediscovered in the 1930s — the entrance had been buried for centuries. Standing inside it, even in the company of other visitors, the sense of antiquity is almost physical.

Above the cave, the acropolis of Cumae offers views across the coast that make the climb worthwhile in themselves. The Temple of Apollo and the remains of the Temple of Jupiter occupy the heights, and on a clear day the islands of Procida and Ischia are visible offshore.

Underground Naples: The Tunnels, Cisterns, and Catacombs Below the Streets

Beneath the streets of central Naples lies an entirely parallel world. The city sits on layers of volcanic tufa — soft, easily quarried stone that the Greeks began extracting as building material around 470 BC. Over the following two millennia, the quarrying left behind a network of cavities that were progressively repurposed: as cisterns for the Greek and Roman water supply system, as air-raid shelters during World War II, and eventually as dumping grounds for everything the city above didn’t want to look at. Naples Underground is extensive, poorly mapped even today, and partially open to visitors through several different access points.

Napoli Sotterranea, with its entrance near Piazza San Gaetano in the historic center, offers the most comprehensive guided tour of this underground network. The tour descends into the original Greek-Roman cistern system, passing through narrow rock-cut passages and large underground chambers. During World War II, these spaces sheltered up to 10,000 civilians during Allied bombing raids, and remnants of that period — wartime graffiti, furnishings, a small theater built by the sheltering residents — are preserved in one of the chambers. The tour is well-organized and runs in multiple languages throughout the day.

Separately, the Catacombe di San Gennaro in the Rione Sanità neighborhood are the most significant early Christian catacombs in southern Italy, dating from the 2nd century AD. Unlike the Roman catacombs, which are cramped and often underwhelming in person, the Catacombs of San Gennaro are spacious, well-lit, and decorated with frescoes and mosaics that have survived in remarkable condition. Guided tours run from the adjacent basilica and the proceeds support social projects in the neighborhood — a genuinely good reason to visit. The Rione Sanità itself, a historically marginalized quarter that has been slowly revitalized over the past decade, is worth spending time in above ground as well. Its street art, local bakeries, and baroque churches form a neighborhood experience that feels nothing like the tourist trail of Spaccanapoli.

The Bourbon Tunnel (Galleria Borbonica), accessible from Vico del Grottone near Piazza del Plebiscito, is a different kind of underground Naples entirely. It was commissioned by King Ferdinand II of Bourbon in 1853 as an escape route connecting the Royal Palace to the military barracks, in case of revolution — a practical fear for a king ruling a restless city. The tunnel was never completed for its original purpose, but was used as an air-raid shelter in World War II and later as a police vehicle depot. It contains a surreal collection of abandoned vehicles, wartime artifacts, and votive offerings left by civilians who sheltered here. The adventure tour, which involves crawling and rope descents, is particularly popular, but the standard walking tour covers the essential atmosphere without requiring a change of clothes.

The Phlegraean Coast: Lesser-Known Ruins and Views Away from the Crowds

Between Pozzuoli and Cumae, the coast passes through a string of small towns and headlands that most visitors to the Campi Flegrei either miss or hurry through. This stretch rewards slower travel.

At Bacoli, the Piscina Mirabilis is the largest cistern built by the Romans in the ancient world — a vast, cathedral-like underground hall with 48 pillars and barrel-vaulted ceilings, designed to supply fresh water to the Roman fleet based at nearby Misenum. It is enormous, dimly lit, and almost always empty of visitors. Access requires a reservation through the local municipality, which involves a modest fee and advance planning, but the effort is completely disproportionate to the difficulty. You will likely have it to yourself.

Also at Bacoli, the Cento Camerelle — literally “one hundred small rooms” — is a two-level Roman cistern cut into the tufa, originally part of a private Roman villa. The upper level is accessible; the lower level floods seasonally. Nearby, the Tomb of Agrippina (which is probably not actually Agrippina’s tomb, but the name has stuck) is a large Roman funerary building reduced to its brick barrel vault, sitting in an overgrown state near the waterfront. These are the kinds of ruins that exist in almost no tourist infrastructure at all — no signs, no gift shops, no cafes. You find them, you look at them, you leave. That experience has its own value.

Cape Misenum, at the western tip of the Campi Flegrei, was the base of the Roman Empire’s largest naval fleet for three centuries. Pliny the Elder departed from here on the day Vesuvius erupted in 79 AD and died trying to rescue survivors across the bay. The cape today is quiet and residential, with sweeping views back across the bay toward Naples and Vesuvius — a view that makes the scale of the entire Campi Flegrei system tangible in a way that no map quite manages.

Planning Your Visit: How to Get There, When to Go, and What to Know

The Campi Flegrei and underground Naples are best used as extensions of a Naples visit rather than standalone destinations. Naples is well-served by international flights into Naples Capodichino Airport, and by high-speed rail from Rome (about 70 minutes) and other Italian cities. For the purposes of this guide, getting around within the region is the more practical question.

Getting to the Campi Flegrei from Naples: The Cumana railway, operated by EAV, runs from Montesanto station in central Naples along the northern shore of the bay, stopping at Pozzuoli, Lucrino (for Baia and Lake Avernus), and Torregaveta (for Cumae). It is inexpensive, runs frequently, and is the most straightforward way to link the major Campi Flegrei sites. The Naples metro Line 2 also runs to Pozzuoli. A car gives more flexibility for reaching Solfatara, Bacoli, and the Piscina Mirabilis, but traffic in and around Pozzuoli can be frustrating, and parking is erratic.

Underground Naples access: Napoli Sotterranea and the Bourbon Tunnel are both in central Naples and easily reached on foot from the historic center. The Catacombs of San Gennaro require a short taxi or bus ride to Rione Sanità, or a 20-minute walk uphill from the historic center.

When to go: Spring (April and May) and autumn (September and October) offer the best conditions — mild temperatures, lower visitor numbers, and better light for photography. Summer is hot and Pozzuoli and Baia attract Italian day-trippers, though foreign tourists remain relatively scarce. Many of the smaller sites (Piscina Mirabilis, Cento Camerelle) require advance booking regardless of season, so plan those at least a few days ahead.

What to wear and carry: Underground sites are consistently cool regardless of outside temperatures — bring a layer even in August. Some tunnels and passages require crouching or crawling. Closed-toe shoes are essential for Solfatara, where the ground surface is uneven and occasionally warm. The Campi Flegrei sites involve a lot of walking across uneven volcanic terrain, so comfortable footwear matters throughout.

Practical notes on the sites:

• The Anfiteatro Flavio in Pozzuoli is managed by the same ticketing system as many other Campania heritage sites — the Campania Artecard offers good value if you plan to visit multiple sites over several days.
• The Underwater Archaeological Park at Baia requires booking glass-bottom boat tours through local operators in advance, particularly in summer.
• Solfatara has its own entrance fee and is managed independently. Check current opening hours before visiting, as they have varied since the site’s reopening.
• Photography is permitted at most sites but flash is sometimes restricted in the underground tunnels and catacombs.
• The Museo Archeologico dei Campi Flegrei at Castello di Baia is closed on Tuesdays, following standard Italian museum convention.

Where to stay: Staying in central Naples gives you the best access to the underground sites and easy rail connections to the Campi Flegrei. The neighborhoods of Chiaia, Mergellina, and the Spanish Quarter all offer good accommodation at a range of price points and are well-positioned for early morning departures on the Cumana line. Pozzuoli itself has a growing number of B&Bs and small hotels, which makes sense if you plan to spend more than a day on the western side of the bay.

What to avoid: Don’t underestimate distances and travel times between Campi Flegrei sites — they are spread across a fairly wide area and the roads are not always fast. Trying to cover Solfatara, Baia, and Cumae in a single day without a car is ambitious to the point of being counterproductive. Better to group sites geographically and spend two days in the region rather than one exhausting sweep. And don’t skip the food. Pozzuoli’s port restaurants and the small trattorias in Bacoli represent some of the best, most honest seafood cooking in the Naples area, in a context where nobody is pricing for tourists.

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📷 Featured image by Zsolt Cserna on Unsplash.