One by one, the visitors descend through a tight tunnel cut through volcanic rock into the damp foundations of the Teatro Romano buried beneath Herculaneum, with the weight of 2,000 years of city above them. “This is a time machine,” the guide says, “and we are going back.” It is pitch black as film-maker Gianfranco Rosi’s camera finds torchlight catching the tourists’ transparent waterproof capes, making them appear like ghosts.

Released on the streaming platform Mubi this March, Rosi’s documentary Pompei: Below the Clouds threads a needle from classical antiquity to the present day. Presented in ashen black and white, without narration or interviews, it places the viewer inside the region surrounding Naples and leaves us there, each scene presenting a place and a moment in the area’s long history.

Naples is one of the oldest continuously inhabited cities in the world, and most visitors see only a fraction of it before boarding the Circumvesuviana at Porta Nolana and riding the narrow-gauge railway east to Pompei or Herculaneum. In Below the Clouds, Rosi does not alight there. He stays on the train, camera in hand, and traverses this seismic landscape – from the Sorrentine peninsula, crowned by Vesuvius in the east, to the lesser-known craters of the Phlegraean Fields in the west. The train, Rosi says, is “my time machine”. His lens draws us into the Naples most visitors never see.

As a film-maker myself, who has lived and worked in Naples for the past 15 years, I was inspired by Below the Clouds to make my own pilgrimage, and boarded the overcrowded, noisy trains I usually avoid.

Before the Circumvesuviana reaches the archaeological site of Pompei, it skirts the Bay of Naples, passing through a number of overlooked towns characterised by a stratification of history visible in the architecture. Drawing into the station of Torre Annunziata, Rosi holds the camera on the visible layers of the town’s history: diamond-patterned Roman brickwork cut from nearby volcanic quarries, Doric columns from an excavated Roman villa, and the still-lived-in mid-century housing blocks rising above them. That Roman villa is worth stopping for. Believed to have been built for Poppaea Sabina, the second wife of Emperor Nero, Villa Oplontis feels like a secret discovery. Its frescoes are almost untouched, its colonnade pristine, and on this day, as always, there was scarcely another soul in sight.

Back on the Circumvesuviana, I head east to Somma Vesuviana. A team from the University of Tokyo has been excavating here for decades, slowly uncovering the Villa Augustea, the imperial estate where the Emperor Augustus is believed to have died in AD14. It was not the great eruption of AD79 that buried the villa, but a later one in AD472. The archaeological treasures still buried across the region are so numerous that tomb raiders have long burrowed into the soft volcanic stone looking for loot to sell on.

A second train line, the Cumana, runs in the opposite direction. It departs from Montesanto station in central Naples and heads west, reaching Pozzuoli in 25 minutes. At the end of the line lies a working port city of 75,000 people living in the basin of one of the world’s most geologically active calderas (volcanic craters). The lore surrounding Vesuvius has long overshadowed the dangers posed by the Phlegraean Fields, which rumble daily beneath the city’s foundations.

Stepping off the train at Pozzuoli, I was hit by the pungent sulphuric smoke drifting over the port. I had timed my arrival for a simple lunch at Abbascio ù Mare (a local favourite serving fish landed from the boats that morning) before visiting the Macellum of Pozzuoli, a 2nd-century Roman market near the harbour. Here, I found the clearest record of what is known as bradyseism, the movement of magmatic fluid and gas beneath the surface of the Earth that lifts and lowers the land, sinking entire towns and raising them again centuries later. Halfway up the ancient columns, I spotted bands of small holes in the stone. These were bored by molluscs when the columns once stood metres below the bay. Rosi’s camera follows the phenomenon underwater, descending into the submerged ruins of nearby Baia, where robed marble figures stand upright on the seabed as shoals of fish drift over mosaics and between their feet.

Between east and west, at the intersection of the Circumvesuviana and the Cumana, lies Naples – known to the Greco-Romans as Neapolis (the new town) because it was new compared with Pompei and Baia. In the centre of the city, at the Museo Archeologico Nazionale di Napoli, Rosi films Maria, the museum’s archaeologist, deep in the storage vaults. This is what he calls the casaforte (the safe of memory) – shelf upon shelf of fragmented marble torsos, legs and busts, the overflow of 2,000 years of excavation. These artefacts are down here, Maria says, until it is their turn to return to the museum floor above – a mirror, Rosi suggests to me when we speak, of society’s own hierarchies. Like Rosi, I am obsessed with these perfectly formed marble figures, the survivors of catastrophe, that live in the galleries of the museum upstairs among the frescoes and bronzes, pulled from the same volcanic earth that buried thousands of people under Vesuvius.

Rosi juxtaposes the marble torsos with shots of dismembered ex-voto, small metal plates shaped like individual body parts. These are offerings, often left in churches or street shrines along with prayers to saints in exchange for bodily cures.

At the small church of Santa Maria Francesca delle Cinque Piaghe in the Quartieri Spagnoli, one of my favourite corners of the city, hundreds of ex-votos in the shape of pregnant women have been left for the saint of fertility. These practices, still very much alive today, speak to the Neapolitan impulse to marry the sacred and the profane.

Rosi’s film ends in an abandoned cinema somewhere along the train line, its seats destroyed, its screen partly intact. Into this ruin, Rosi projects clips from Roberto Rossellini’s Journey to Italy, a film about the past, playing in a ruin, in a city built on ruins, above a city that was itself once buried. Like a Chronovisor (a mythical 1950s invention that supposedly broadcast actual historical events), the cinema is where the present tense becomes the past even as you watch it. Just like Naples. Just like Below the Clouds.

By the end of the film’s nearly two-hour runtime, the viewer has made the same journey as those visitors descending into the foundations of the Teatro Romano in Herculaneum to behold and reflect on a civilisation buried mid-sentence. Below the Clouds insists, however, that this confrontation does not require a museum ticket. “We are already living inside the catastrophe,” says Rosi.

Pompei: Below the Clouds is available on Mubi. Herculaneum, Pompei, Villa Oplontis, Museo Archeologico Nazionale di Napoli are open to visitors. The Circumvesuviana runs from Napoli Porta Nolana east to Pompei and Herculaneum. The Cumana line runs from Montesanto station west to Pozzuoli. Sophia Seymour offers bespoke city walks and itineraries through Looking for Lila