“Can you guess which way it is to Piazza di Spagna from here?” says Davide d’Alonzo, our guide and technical manager at Acea, Rome’s water-management company. His face is illuminated in the darkness by a large flashlight. “The answer is that you have to look at the way the water is flowing,” he says, pointing into the murky tunnels in the direction of Rome’s landmark square. In truth, it’s not easy to get one’s bearings. 

Only moments ago, we had left our personal belongings behind (save for my Monocle notepad and pen), donned white hard hats, dark rubber suits and Wellington boots, and stepped through an inconspicuous door near Rome’s Villa Medici. An impressive spiral staircase takes us down 112 steps to a tunnel 25 metres below ground. We find ourselves beneath the Italian capital’s streets and inside one of Europe’s oldest functioning aqueducts, the Acqua Vergine, whose water has been celebrated throughout the ages for its purity. Dating from 19 bce, it is a masterpiece of classical engineering and Monocle has been granted rare access. 

Originally used to meet the needs of the metropolis’s noble families, the aqueduct continues to supply water to the city’s parks and gardens, and performs a task that visitors particularly appreciate: replenishing some of Rome’s most important fountains, from those on Via del Corso to the one in front of the Quirinale presidential palace. As we wade waist-deep through the water, D’Alonzo instructs us to walk slowly without lifting our feet too much. The reason for caution, he explains, is that we are not only moving against the current but also because the water, after travelling for some 20km from its source east of the city, comes out at Rome’s famed Trevi Fountain. And we wouldn’t want to muddy the waters, would we? 

Being here is a privilege: while a phalanx of tourists is queuing above ground to take selfies in front of the crystal-clear waters and sculpted travertine of the famed fountain, we are down here with just a couple of guides and our photographer. The only sound is the water dripping from the tunnel’s roof, stalactites hanging overhead. Suddenly we hear a disconcerting rumble that wrests us away from our Roman reverie. “Don’t worry, that’s just the metro,” says D’Alonzo, pointing to parts of the aqueduct that have been strengthened with concrete to counter this relatively recent addition to the city’s subterranean infrastructure. As we wander slowly beneath Rome, D’Alonzo schools us in the history of the Vergine. It was built in just two years by the Romans, who relied on a series of wells for the water supply. 

Our day comes to a close with a visit to the water station by the Trevi Fountain, where the aqueduct ends. Inside the building, 18th-century pipes are still visible, their size and pressure dependent on each noble family’s status and ability to pay.

From our vantage point behind the fountain, we look through little windows to see people gathered on the other side of the water. And yet there’s no doubt where I would rather be: contemplating the unseen side of this famous city and the slow passage of the aqueduct’s watery cargo on its 23-hour journey to one of Rome’s busiest squares. The ancient Vergine, quite literally, keeps the city moving.