Long stretch of underground aqueduct found in Naples

A previously unknown subterranean tract nearly half a mile long of an Augustan-era aqueduct has been rediscovered by spelunkers in Naples, southern Italy. Spelo-archaeologists from the non-profit Cocceius Association were able to investigate 647 meters (2123 feet) of a branch that tunnels through the Posillipo Hill.

The Aqua Augusta was unique in the Roman world because it supplied not just one main urban nucleus, but at least eight of them in the Bay of Naples area, including Naples, Pompeii, Herculaneum and Cumae. It was built between 33 and 12 B.C by Marcus Vipsanius Agrippa, best friend and son-in-law of Augustus, when he held the high rank of curator aquarum, the magistrate in charge of the supply, management and maintenance of Rome’s aqueducts.

The source of the water was the town of Serino in the Campanian Apennines which was famed for the purity of its springs. The aqueduct was more than 90 miles long and consisted of 10 branches, seven leading to urban centers and three private offshoots carrying water to the villas of the wealthy. It would remain the longest Roman aqueduct for more than 400 years until construction of the 265 mile-long Aqueduct of Constantinople.

Most of the route was outdoors through channels and brick arches, but the mountainous terrain often required the creation of underground tunnels through a variety of rock types. Tunnels were dug directly into soft rock like tufa or limestone, but sandy soil required the construction of walls and barrel vaulting. The remains of a stretch of brick arches can be found in the Ponti Rossi area of Naples. The endpoint also still stands. The Piscina mirabilis, a massive barrel vaulted cistern built in the port city of Misenum to provision the naval fleet housed there with fresh water, is the largest ancient Roman cistern in the world.

Despite its significance as one of Rome’s greatest architectural achievements, the Aqua Augusta has been little explored and barely documented. The existence of an aqueduct length under Posillipo was noted in 1938 by doctor and archaeology enthusiast Italo Sgobbo, but he did not document it. There was no map or description of the location of the entrance or of the tunnels.

In 2021, Neapolitan native Giuseppe Scodes informed the Cocceius Association that he and his friends had played in those ancient channels as children 40 years ago. Last year the organization was able to secure the permits to access the potential site and Scodes led them to where he remembered the entrances once had been. They were completely covered, however, obscured and inaccessible thanks to forest growth in the decades since young Peppe had played in the coolest fort a kid could ask for.

After days of clearing the underbrush, they were able to reach the tufa wall and climb 13 feet up its vertical surface to get a look inside a hole. That was definitely an ancient access point to the aqueduct, but it was impracticable as an access point for a modern exploration team. They kept looking for an easier ingress, and luckily they found one 250 feet away. After that all they had to do was spelunk, and that’s the easy part. (To them.)

They found the aqueduct tunnels in excellent condition. The section they explored channeled water from the Crypta Neapolitana (originally a military communications tunnel) to Posillipo and the neighboring island of Nisida. It consists of one main specus (roofed channel) 28 inches wide at the roomiest part. A layer of hydraulic plaster 25 inches high was found at the base of the walls, and it is in turn coated in a thick layer of limestone deposited by centuries of flowing hard water. It is a winding, turning path, the result of errors and/or necessary derailments encountered by Agrippa’s builders.

There are some blockages and tight spots, but the full length is explorable. This is the first long uninterrupted stretch of a Roman aqueduct in outstanding condition that will be able to be studied in minute detail.

Rabun Taylor(opens in new tab), a professor of classics at the University of Texas at Austin who was not involved in the report, told Live Science in an email that the newly discovered aqueduct section is interesting because it is “actually a byway that served elite Roman villas, not a city. Multiple demands on this single water source stretched it very thin, requiring careful maintenance and strict rationing.”

Taylor, an expert on Roman aqueducts, also said the new find “may be able to tell us a lot about the local climate over hundreds of years when the water was flowing.” This insight is possible thanks to a thick deposit of lime, a calcium-rich mineral that “accumulates annually like tree rings and can be analyzed isotopically as a proxy for temperature and rainfall,” he explained.

Ferrari, Lamagna and other members of the Cocceius Association plan to analyze the construction of the aqueduct as well, to determine the methods used and the presence of water control structures. “We believe that there are ample prospects for defining a research and exploration plan for this important discovery, which adds a significant element to the knowledge of the ancient population” living in the Bay of Naples, they wrote in the report.