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Baiae: A Roman settlement at the bottom of the sea

Today, a team of archaeologists and engineers are developing some surprising new technologies to protect the underwater site for future generations.

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“You’re sure I can cross?” I had to almost shout to be heard. Wooden slats dotted the ground before me. About 30m to my right, steam rose into the sky in thick grey-white clouds. And somewhere between where I stood now, and there, the earth turned from solid and cool to boiling and viscous. Wherever that exact change happened, I wanted to make sure I was none too close.

It’s very dangerous here

Sì, sì,” said volcanologist Enzo Morra, my guide for the day. He was already climbing the hill on the other side of the wooden slats before me.

I edged one foot onto one piece of wood, then the next. The ground felt firm. As I reached the far side and climbed the hilltop, I could see the source of the steam: a bubbling pool of dull gunmetal-grey mud, ominous as the contents of a witch’s cauldron and a great deal louder. The air smelled of sulphur.

“It’s very dangerous here,” Morra welcomed me when I arrived. “More dangerous than Vesuvius.”

Campi Flegrei is one of 20 known “supervolcanos” on the planet (Credit: Amanda Ruggeri)

Campi Flegrei is one of 20 known “supervolcanos” on the planet (Credit: Amanda Ruggeri)

I laughed nervously. “I wish you’d told me that when we were over there. Why are you telling me that when we’re here?”

We were overlooking one of the fumaroles of Campi Flegrei, known in English as the Phlegraean Fields. One of 20 known “supervolcanoes” on the planet – capable of erupting with a volume thousands of times stronger than an average volcano – Campi Flegrei commands less notoriety than Mt Vesuvius, just 30km to the west. But that is largely down to luck. If Campi Flegrei were to blow at maximum capacity today, it would make the 79AD eruption of Mt Vesuvius that destroyed Pompeii look like a puppy’s sneeze. Fortunately, Campi Flegrei hasn’t had a full-force eruption in thousands of years.

That isn’t to say it’s impossible. Researchers call the supervolcano “restless”, and there are concerns it is becoming more so. In 2012, the alert level was raised from green to yellow, indicating a need for more monitoring. Most recently, a “seismic swarm” in April 2020 saw 34 different earthquakes.


Campi Flegrei is more than a (fitfully) snoozing menace. It’s why the ancient Romans built one of the most magnificent resort towns on the Italian peninsula here: Baiae, famed for its hot springs and bad behaviour. It’s also why at least half of the town, with its precious marbles, mosaics and sculptures, sank beneath the Mediterranean over the following centuries. Now, this “restless” supervolcano is the reason why much of this archaeological site is at risk today – both indirectly, thanks to the sea’s effect on the artefacts, and directly, in terms of the threat of earthquakes or another volcanic eruption.

The Romans had few ways of knowing when an eruption or earthquake was coming. They were all but helpless when it came to protecting their town against the encroaching sea. But that’s no longer true. Today, a team of archaeologists and engineers are developing some surprising new technologies to protect the underwater site for future generations. And that’s what I’ve come here to learn more about.

Lured by the volcano's hot springs, the Romans built the magnificent resort town of Baiae here (Credit: Amanda Ruggeri)

Lured by the volcano’s hot springs, the Romans built the magnificent resort town of Baiae here (Credit: Amanda Ruggeri)

Over its full 13km radius, the supervolcano, almost all of it at ground level or beneath the sea, has 24 craters and more than 150 pools of boiling mud. It’s easy to see how the ancient Greeks, who settled here first, came up with the name: “Phlegraean Fields” is from the early Greek verb phlégō (“to burn”).

The danger of Campi Flegrei isn’t just its size and strength, but its randomness. When a volcano like Vesuvius erupts, you know where the eruption will come from: the cone at its peak. Not here.

“The activity isn’t ever in the same place. Every eruption has its own story and place of emission,” Morra said. “Therefore, we obviously don’t know when the eruption will happen. But we also don’t know where the next eruption will happen, if there is one.”

Another danger is the type of activity: more than 90% of the activity Campi Flegrei is explosive, not effusive. In other words, when it blows, it won’t leak lava over the ground; it will punch a column of rock and lava into the air. When the detritus lands, the ash will blacken the sky and thicken the air, making both seeing and breathing near-impossible. The column’s collapse causes a pyroclastic flow: extreme heat of up to 700C that vaporises everything in its path.

That, at least, is what happened 39,000 years ago, the date of Campi Flegrei’s largest eruption. Molten rock spewed 70km high. Ashes were found as far away as Siberia. The explosion was so powerful, the volcano collapsed into a caldera. The cooling that occurred in the ensuing years may even have helped bring about the end of the Neanderthals.

Fifteen thousand years ago, Campi Flegrei erupted again. The eruption wasn’t as large, but it threw significant volumes of yellow tufa into the air – enough to give Naples its colour today. People carved through and built with the local stone, giving the palazzi, churches and even underground tunnels their golden colour.

The last significant eruption was in 1538. Compared to these previous two events, it was tiny. It was also big enough to throw ash and pumice 5.5km high. As the column collapsed, it created a “new mountain” (dubbed, quite literally, Monte Nuovo), measuring 123m high – and burying a village beneath it. If this happened today, in the vicinity of Italy’s third-most-populous city, Naples, the damage would be severe.

So what is the possibility of such an eruption happening in our lifetimes?

“Obviously we can’t make estimates,” Morra said, almost languidly. “We know that an active volcano, any active volcano, can erupt. Clearly, in our heart – we hope not.” I looked worried. “Have courage!” he said. “Like Vesuvius, Campi Flegrei is continuously monitored by colleagues at the Vesuvian Observatory, the oldest volcano observatory in the world. This can make us feel more tranquil.”

Close monitoring means an eruption can be predicted months in advance. With enough warning, the hope is that the metropolitan area can be safely evacuated.

Signs of a pending eruption aren’t the only data that volcanologists collect. The Vesuvian Observatory was also the first to discover, and chart, a phenomenon known as “bradyseism”: the slow rising, and sinking, of land over time. As the magma in Campi Flegrei’s massive magma chamber moves 3km below ground, so does the land above – sometimes significantly. Over the last 15,000 years, the movement of the magma has pushed the land above it upward by some 90m. At the same time, other parts of the caldera have fallen.

As a result, like Vesuvius, Campi Flegrei has given the area around it much of what makes it special: its volcanic rocks, soft and easy for building; its volcanic soil, rich with nutrients for vineyards and lemon groves; even the crescent shape of its coast, providing a gulf for splashing and sunning.

But what the supervolcano has given the area, it also can take away – even without an eruption.


On the eastern edge of the caldera, the above-ground archaeological site of Baiae overlooks the sea. A layer-cake of arches, walls and terraces, it was once the ultimate holiday spot for rich and aristocratic Romans, a kind of Las Vegas of the ancient world. Now stripped of most of their marble, frescoes and sculptures – many of which are now at the Archaeological Museum of Campi Flegrei – the buildings look little like they would have millennia ago. Graceful capitals, shorn of their columns, and stucco decorations, dotted with cherubs and swans, hint at its former opulence.

The archaeological site of Baiae was once the ultimate holiday spot for rich and aristocratic Romans (Credit: Amanda Ruggeri)

The archaeological site of Baiae was once the ultimate holiday spot for rich and aristocratic Romans (Credit: Amanda Ruggeri)

As I walked through the site with University of Naples L’Orientale archaeologist Michele Stefanile, he pointed out to me what each structure once would have been: a villa, a bath, a theatre. In one room, I tiptoed around the red and white mosaics. In another, we admired the wall frescoes, still vibrant with ochre and crimson.

The Romans came here for the same reasons we do: the sparkling Mediterranean, the balmy weather, the lush vegetation. They were also drawn to the area’s thermal springs – the result, of co urse, of the volcanic activity beneath their feet. When Baiae first entered the historical record in 178BC, it was as the Aquae Cumanae (Cumaean Waters).

But Baiae wasn’t just a spa retreat. It was a party town, a place for Romans to bathe and banquet, flirt and frolic. In one of his many elegies to his lover and muse Cynthia, even the poet Sextus Propertius, no great prude, wrote despairingly in 25BC:

“But you must quickly leave degenerate Baiae;
these beaches bring divorce to many,
beaches for long the enemy of decent girls.
A curse on Baiae’s water, love’s disgrace!”

As the Roman Republic lapsed into Empire, Baiae’s reputation only grew. In 39AD, Caligula built a bridge – made up of merchant ships linked together, then covered with earth – from Baiae to Pozzuoli, three miles long, then rode over it in a chariot. In 59AD, Nero had his mother, Agrippina, murdered in her villa here. The later emperor Hadrian would have a more peaceful end to his life, dying of natural causes in his Baiaen palace in 138AD.

More straight-laced Romans stayed away, or claimed to. “It’s no wonder that men like Seneca, for example, decided to have his villa not in Baiae but in a hill in that direction – just to stay a little bit isolated,” Stefanile told me, pointing across the gulf. Even towards the end of his life in 65AD, the philosopher said: “Baiae is a place to be avoided, because, though it has certain natural advantages, luxury has claimed it for her own exclusive resort.”

A vast array of sculptures and inscriptions, now on display at the Archaeological Museum of Campi Flegrei, highlight Baiae's former opulence (Credit: Pomona Pictures)

A vast array of sculptures and inscriptions, now on display at the Archaeological Museum of Campi Flegrei, highlight Baiae’s former opulence (Credit: Pomona Pictures)

“Luxury” was right. Not content with building on land, wealthy Romans erected pylons and built their villas directly over the sea itself. Horace, rather more prudish than Propertius, reprimanded his countrymen in 23BC for the immodesty of such actions:

“You, on the brink of the grave,
contract for the cutting of marble slabs;
forgetful of death you fret
to build your mansion out from the coast
in the roaring sea at Baiae –
the mainland shore will not suffice.”

Horace’s reprimands aside, the effect would have been magnificent. “All of these villas, complexes and structures were conceived in order to be seen from the sea,” Stefanile said as we stood on a terrace. Beyond the ruins lay a clutch of pastel homes; past that, the glittering water. In the near distance, the slopes of Mt Vesuvius were purple in the summer haze.

“We always make the mistake of putting ourselves on the ground,” Stefanile said. “But the perfect point of view to appreciate this is the sea. Just imagine being in the gulf of Baiae and seeing this leisure resort with all these terraces and the pools and the people crowded here.”

Even as the western Roman Empire declined, Romans, and then Visigoths and Vandals, continued to use the baths at Baiae. But by the time Giovanni Boccaccio described it in a 1344 novel – “no sight under the sun is more beautiful or more pleasant than this”, he wrote – Baiae’s great baths and villas had fallen into ruin.

The underwater statues at Baiae are replicas but still feel "ghostly and sublime" (Credit: Pomona Pictures)

The underwater statues at Baiae are replicas but still feel “ghostly and sublime” (Credit: Pomona Pictures)

Because of bradyseism, many were also underwater. Over the last 2,000 years, much of the site has sunk between 4-6m; in some places, it’s up to 10m. About 50% of built-up area is now thought to be under the sea.

Some of the artefacts were covered by sand, hiding them from humans and animals alike. But others weren’t so lucky. There are stories of fishermen casting their nets and hauling in ancient sculptures, and of precious objects passing into the black market. Because no-one could be sure exactly how many objects were actually within the site, no-one can be sure how much has been looted.

This is not a normal archaeological park

In 2002, the 177-hectare underwater site was made a Marine Protected Area. While licensed scuba divers can explore the site, they must do so with one of the registered local dive shops and guides, who see themselves as the first defenders of their heritage. Today, archaeologists are less worried about looting. But other challenges remain.

“This is not a normal archaeological park,” Stefanile said. “You cannot put a fence around it. You cannot close it. It’s always open. And it’s exposed to marine life, to the waves, to the tides, and to the human presence.”

Barbara Davidde, Stefanile’s boss and the director of the unit of underwater archaeology at Italy’s Central Institute for Restoration, has been working at Baiae since 1993. One of the main problems for the artefacts underwater, she says, is marine life. Bacteria, bivalves, sponges – a dizzying variety of organisms not only make their home in the sea but have a penchant for stone and marble materials.

“If you leave these artefacts open (for example, uncovered by sand), the marine organisms immediately start to colonise, and to live on, the surface. They start to destroy and attack the materials,” she said. Later, at her lab in Rome, Davidde showed me what she meant: while a mosaic might look undisturbed to the naked eye, under a microscope each was a web of holes and divots.

At the Archaeological Museum of Campi Flegrei, one piece of artwork after another shows the damage animals can do. While it’s common to see ancient Roman statues missing arms or heads, the items in this collection are different. A veiled woman has been so disfigured, she looks as formless as a ghost; a base with a dedication to Emperor Hadrian loses all shape at the top, like a half-melted candle.

My favourite, however, is a 74cm statue of Zeus on his throne. Dating to the 1st Century BC, his right side is covered with what looks like white scrawlings, the remnants of marine encrustations. Holes that start to dot his torso turn his right arm into a handless sponge. His other side – which was presumably buried in sand – is practically pristine, the folds of his toga still sharp.

Marine organisms attack the statues left underwater — damage seen with this enthroned Zeus (Credit: Pomona Pictures)

Marine organisms attack the statues left underwater — damage seen with this enthroned Zeus (Credit: Pomona Pictures)

The Zeus was likely stolen by looters. He wound up in the collection of the J Paul Getty Museum in Los Angeles, acquired by a curator later indicted for conspiracy to traffic in illegal antiquities. He was returned to the Campi Flegrei museum in 2018. The other sculptures in the museum are now safe from marine organisms. But the artefacts that remain below the sea, including the precious floors that make up the largest collection of underwater Roman mosaics the world, remain at risk.

We have to find a way to protect the site

“I don’t think that you can find any other archaeological site in the world like Baiae,” Davidde said. “We have to find a way to protect the site.” Musas, an interdisciplinary project led by Davidde to adapt new technologies for underwater sites, is doing just that.


Located in the shadow of Monte Nuovo, the Centro Sub Campi Flegrei dive centre was bustling on the day I was there. A dozen researchers, engineers and archaeologists were setting up equipment – which today included not only scuba tanks and gear, but waterproof tablets, cables and even an underwater drone.

Overseeing it all was Chiara Petrioli, professor of computer science at the University of Rome La Sapienza and Musas’s scientific coordinator. She is behind one of the more ambitious aspects of Musas: its underwater wireless sensor networks.

When it comes to an underwater site, a major challenge is communication. The various networks we rely on above ground – data, wifi, radio – aren’t effective in water. Wifi requires laying cables and penetrates only a couple of centimetres. Wireless optical is better but can cover only a few metres of range.

Underwater drones help scientists above the water to “see” the relics below (Credit: Pomona Pictures)

Underwater drones help scientists above the water to “see” the relics below (Credit: Pomona Pictures)

The impact on how much archaeologists are able to learn about the site, as well as how to best protect it, is enormous. Imagine you’re an underwater archaeologist excavating. Say you need a new tool. You have to rise to the surface, request it, hope they have it on the boat, and take it back down. The back-and-forth is time-intensive, and more dangerous.

Perhaps, after all that, you find a new mosaic. You start to uncover it, but all you can do is jot basic notes on a rudimentary board, perhaps take some photographs with an underwater camera. If you want to confer with anyone else, you have to wait until you’re above water. Without precise GPS, it’s also difficult to pinpoint the site’s location. When you come back a day later, the sands may have shifted, the sea floor changed. How can you be sure you’ll find it again?

The answer to all of this, scientists have found, is to try to mimic how marine mammals communicate: through sound waves.

This underwater mosaic was once part of a floor of a magnificent villa (Credit: Pomona Pictures)

This underwater mosaic was once part of a floor of a magnificent villa (Credit: Pomona Pictures)

“You need to use acoustic communication,” Petrioli said. “This is really challenging, because the sea’s parameters may change.” Just as external factors can disturb the communication between killer whales or dolphins, the same is true for humans using acoustic communication, too. Temperature, salinity and wind all can affect the connection between two devices. So can other sounds – a ship passing by, a jet ski.

“It’s very complex, but we came up with an idea. Let’s have mesh networks, like multi-rope networks, and let’s use artificial intelligence techniques to keep changing the protocol we use.” If a telephone cable sends a message from A to B along a straight line, a mesh network is like a web – in this case made up of underwater wireless sensors, or nodes. When communication is sent, there are various ways it can get from point A to point B – allowing for the message to find the most efficient path to its final destination. And as the sea’s parameters change, so would the communication system. This week, the method they were testing allowed for communication over a range of up to 2km.

The underwater wireless sensor networks open up a number of possibilities. Divers now can communicate in real-time, both with one another and with people above the surface, using waterproof smart tablets. Teams can pinpoint exactly where the diver – and a particular site or artefact – is located. The networks even allow data to be gathered, in real time, about the conservation status of the site – including sending images up to experts on land. And by monitoring everything from water quality to CO2 levels, they also provide extra information about the volcanic activity of Campi Flegrei.

The team was testing all of the technology when I visited, with their hope to install the sensors permanently by summer 2020. While that’s been delayed due to the global coronavirus crisis, there is still hope it will happen this year, and not just in Baiae: Musas has gotten the go-ahead to unroll the same technologies at other ancient underwater Italian sites in Puglia and Ponza.

But for now, I wasn’t sure what seemed more thrilling: the chance to finally see the underwater mosaics and ruins I’d heard so much about, or to try my hand at a technology that allowed divers to communicate like dolphins.

Divers can communicate in real-time with people above the surface using waterproof smart tablets (Credit: Pomona Pictures)

Divers can communicate in real-time with people above the surface using waterproof smart tablets (Credit: Pomona Pictures)

Later that day, I back-rolled into the sea after dive guide Enzo Maione. Descending about 5m deep, we swam over a wall that once made up part of a villa. It was odd to see a ruin like this, grown over with seaweed instead of moss, fish flitting around the bricks.

We were both equipped with tablets. I looked at mine. “Test,” came a message from the boat. “Test OK,” I typed back.

As we swam, statues began to appear out of the misty blue water. I paused, entranced. This was the Nymphaeum of Emperor Claudius, a place where the 1st Century AD ruler would have strolled and admired statues. The statues here today are replicas; the originals have been moved on land for safe keeping. But it hardly mattered. Hovering in the water, looking at the statues’ faces as fish darted around us, felt ghostly and sublime.

I pressed a button on my tablet and held it up. A 3D version of how the nymphaeum would have looked appeared onscreen. This was one of the other main goals of Musas: to help divers understand the ruins they were looking at.

But the best was yet to come. Maione stopped on the sea floor and started to push the sand away. With each sweep, more of a mosaic revealed itself until we were hovering over an intricate pattern of circles and hexagons. This was once the floor of a room just off the atrium of a magnificent villa, dating to the 2nd Century AD.

We’d already been down long enough that the team on the boat wanted to check in. “All OK?” I sent back a smiley face.

As we were reaching the end of our dive – an inlaid marble floor and a second mosaic later – I looked up in surprise. A large, cubic drone had descended through the water. It began to follow us, its headlights turning to look at me so seemingly intently, I felt like I’d fallen into a Pixar movie. This was one more tool in the team’s arsenal: a way for those above the water to “see” the relics below.

Volcanic vents made the water warm — and look almost oily (Credit: Pomona Pictures)

Volcanic vents made the water warm — and look almost oily (Credit: Pomona Pictures)

At our final stop, the water looked strange: in small columns out of the ground, rising almost like a flame, it looked thicker, almost oily. A volcanic vent. I put my hand over it: the water was very warm.

It was one more reminder that we weren’t just diving in an ancient Roman resort, but a supervolcano – a volcano that could sweep all of this away in a moment’s work.

What was not possible before is now possible

But the reason we were here at all was down to human innovation. And it was that innovation – and persistence – that now gave Baiae the chance to exist many generations into the future.

I remembered what Petrioli had told me up on land, overlooking the boats being stacked with scuba tanks and cables and tablets. “Many people told us continuously, ‘You will not make it’,” she said. “We are proving them wrong.

“What was not possible before is now possible.”





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