Discovering Roman ruins and underwater towns in the Gulf of Naples

Next time someone tells you to go to hell, don't take it badly. It can be a wonderful trip, and they'll probably want to visit after you. As long, that is, as you go to the Phlegraean Fields, a land of dozing volcanoes, ancient myths, Roman ruins and underwater towns.

It was roamed by Odysseus, Aeneas and the Roman emperors, who turned it into their favourite spa resort because of the healing power in its bubbling and sulphurous waters, and "Nero's Stoves" remain a popular thermal destination today. The natural blazes and 40 craters are the reason the ancients called it Phlegraean – the Greek for "burning". These fields of fire were also known to be the Mouth of Hell, the spot where you could descend into Hades.

Forget the Amalfi Coast, Sorrento and Positano; this is Naples' most exciting satellite kingdom. My friend Angela, a born and bred Neapolitan, always says: "If you go to Naples and skip the Phlegraean fields, you've seen hardly nothing."

So one warm weekend I decide to go into this infernal region, guided by Giovanni, a crazy, down-to-earth taxi driver whom I pick as my personal Virgil. The first place he drops me off is at the Campi Flegrei diving centre. As soon as I plunge no more than five metres below the surface I'm rewarded with a matchless sight.

Imagine a marine Pompeii, a Roman town buried not by ash but by water, submerged whole, with roads, well-kept aristocratic villas, gardens, pillars, mosaic-floors and sensual statues. This is the ancient coastal resort of Baiae, now an archeological park, drowned by a constant, subtle volcanic activity known as bradyseism, that slowly dunked the land below the sea. It strikes me all of a sudden that Mount Vesuvius is right above my head.

My underwater guide points at the "Nymphaeum" of Emperor Claudius, a personal spa, jacuzzi and sauna attached to an impressive banqueting room. The statues, including one of Odysseus himself, are covered in reddish-yellow seaweed and look almost alive. A paved road leads to two villas belonging to Roman aristocrats.

The next dive is to "Portus Julius", a port that Agrippa built for the most important Roman fleet during the civil war between Octavian and Sextus Pompey in 37BC. There are a total of five archaeological sites inside the marine park. The "Pisonian Villa", belonging to an aristocrat who made the terrible mistake of conspiring against Emperor Nero and was ordered to commit suicide for his disloyalty, features a garden with arcades and thermal baths. The most spectacular of all is "Villa Protiro" with the remains of a tavern, a colonnade and black and white mosaics that stand out from the dark sand.

A little deeper, at 16 metres, I spot the "Smoky Reef", named for the active fumaroles hidden between ancient pillars and sending up plumes of sulphurous bubbles from the sea bed. I'm practically diving inside an active crater.

The next day, the land journey starts and Giovanni gives vent to his oratory skills. He spins me the region's mythological tales, weaving in a bit of Neapolitan folklore. First stop is Cape Miseno, a tiny peninsula jutting into the Gulf of Naples. Beach bars and summer villas dot the hill.

On the very top lies the tomb of Misenus, comrade of Aeneas, who fled the Greeks' sack of Troy. According to Virgil's Aeneid, Misenus met his end by committing the sin of arrogance against the gods: he challenged Triton to a musical contest on the conch shell, and the incensed sea god drowned him for his impudence. Aeneas buried his friend here before sailing northward to father the Roman people.

Somewhere between the open sea and Cape Miseno is one spot claimed to be where Odysseus, according to Homer, had to plug his ears against the fatal song of the Sirens on his long journey home to Ithaca.

I am not going that far. The next stop on my own little Odyssey is the volcanic Lake Avernus, believed to be the exact entrance to Hades. This is where Odysseus descended into the underworld to summon the blind prophet Tiresias to foretell his future, and where he met his dead mother and his dead comrades from the Trojan war.

Aeneas made the same descent to see the shade of his father Anchises, and was nearly tipped out of the ferryman Charon's boat because he was still alive. It's on this lake that "Portus Julius" was built before it sank beneath the waves. The way mythology intertwines with history leaves me awestruck.

I'm in a reverie when Giovanni pulls-up at the Solfatara: a sleeping volcano with a sulphur-yellow sand crater and steam coming out of it which the Romans believed to be the house of Vulcan, the god of Fire. Locals come to picnic here on Sundays, and nobody, since Roman times, has ever been scared of the vapours. Actually, they're a good sign. "As long as the Solfatara vents out its anger, we can be sure and safe that the Vesuvius will not erupt. It's like a pot on the gas," explains Giovanni.

We drive on to the ancient cities of Bacoli and Pozzuoli, where pastel-colour buildings mix with ancient architecture and palms. Grass-covered temples and theatres pop out of nowhere, even in the middle of public squares. They seem to come out from the ground. It's an open-air museum. In Pozzuoli, the Temple of Serapis, dating back to the 1st century AD, used to be a marketplace built around a statue of an Egyptian god. It lies several metres below ground level. Giovanni points at the marble pillars: "You see how volcanic activity has lowered the earth, look at those green fossils covering the stones. Those were molluscs, this place used to be sea."

The boundary between land and sea seems as unstable as that between history and myth: each becomes the other. Bewildered, I go searching for answers to – where else? – Cumae, the cave of the famous Cumaean Sibyl: it's a long, winding tunnel, and at the end of it are a crowd of Neapolitans who have come to put questions to the oracle. Despite the absence of any response from the Sibyl, it says much about this place that, after all these centuries, local people are still coming here to ask.