About 30 meters above sea level, 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. Where that exact change happened, I wanted to make sure I was none too close. It’s very dangerous here,” said vulcanologist 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 onto 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 as great dealer. 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 “supervolcanoes” on the planet I learned nervously. “I wish you’d told me that when we were there. Why are you telling me that when we’re here?”

We were overlooking one of the craters 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’s largely down to luck. If Campi Flegrei were to blow at maximum capacity today, it would make the 79 AD 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 significant earthquakes.

Campi Flegrei is more than a (figuratively) 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 seaside beauty.

It’s also why at least half of the town, with its precious marble, 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 artifacts, 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 from 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). 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 phlégo̱ (“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.”

Fifteen thousand years ago, Campi Flegrei erupted again. The eruption wasn’t as large, but it threw significant volumes of yellow tuffa into the air – enough to give Naples its collar today. People carved through and built with the local stone, giving the palaces, churches, and even underground tunnels their golden color. 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), rising 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 reluctantly. “We know that an active volcano, an active volcano, can erupt. Clearly, in our hearts – 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 volcanic observatory in the world. This can make us feel more tranquil.”