ROME — In a verdant valley east of Rome, Fabrizio Baldi admires a forgotten stretch of a two-tier Roman aqueduct, a stunning example of the emperor Hadrian's 2nd century drive to divert water from rural springs to his ever-thirstier capital.
But Baldi, 36, is less interested in the graceful arches than in where the aqueduct's span ends, hidden in a wooded slope across a stream, halfway up the side of the valley. Scrambling through thick brambles, he comes across a large hole in the ground that appears to be the start of a tunnel.
"Hop in," he says. "This is where the water poured off the aqueduct and started a 21-mile underground journey to Rome."
Baldi is one of about 80 amateur speleologists who spend their weekends crawling down underground channels with laser scanners and GPS in an effort to conclusively map the city's network of 11 ancient aqueducts for the first time in modern history. In doing so, they have turned up underground stretches that nobody remembered.
The group, which has been exploring underground Rome since 1996, has completed about 40% of its mission to map the aqueducts.
"The famous arched, over-ground aqueducts we see today are just the tip of the iceberg; 95% of the network ran underground," says Marco Placidi, head of the speleologists group, which is sharing its results with Italy's culture ministry.
Slaking the thirst of the fast-growing imperial capital meant linking it to springs many miles from the city. The ancient Roman engineers were equal to the task, supplying a quantity of water that modern engineers didn't manage to match until the 1930s.
Rome's emperors had the aqueducts built quickly, employing thousands of slave laborers. In the 1st century, Claudius completed his 60-mile effort in two years.
The structures are unusually solid, with cement and crushed pottery used as building material. One of the aqueducts, the Aqua Virgo, is still in use today, keeping Rome parks and even the Trevi fountain supplied. Others were damaged by invading German tribes in the waning days of the empire.
The ingenious use of gravity and siphons to accelerate water up slopes has stood the test of time: Aqueducts built in the 20th century to supply Los Angeles with water relied on the same methods.
"Interest in what the Romans did underground is growing fast," Placidi says. "Experts now understand they are the best-preserved remains and truly reveal how the Romans made things on the surface work. This is the new frontier of archaeology."
Dropping into the hole, Baldi disappears down the Anio Vetus aqueduct, a 3-foot-wide, 5-foot-high tunnel lined with pristine Roman brickwork. As frogs, spiders and grasshoppers scatter, Baldi reaches a maintenance shaft, complete with good-as-new footholds dug into the bricks that lead up to a narrow opening in the woods 10 feet above. Beyond him, the tunnel vanishes into the darkness.
"Some of this walling is a meter thick and tougher than the rock itself, which is why it has lasted," he said.
The tract of the Anio Vetus aqueduct was mapped by British archaeologist Thomas Ashby, whose 1935 book, "The Aqueducts of Ancient Rome," remains a bible for the cavers.
"But Ashby just followed the maintenance shafts along the surface and didn't get down underground, so where there are no shafts, we are finding things he didn't," Placidi said.
That includes an underground stretch, just over half a mile long, of the Anio Vetus dating to the 3rd century BC that fell into disuse when Hadrian spanned the valley with his arched bridge in the 2nd century.
At nearby Gallicano, the team stumbled on an unknown 300-yard stretch of aqueduct burrowed through a hillside with vertical access shafts ingeniously rising into a second maintenance tunnel above it, large enough for cart traffic.
"We have found Roman dams we didn't know about, branch lines taking water to waterfalls built in private villas, and even aqueducts driven underneath" streams, Placidi said. "We are able to get up close and [feel we are] right back at the moment the slaves were digging."
The explorers say they have no fear because they proceed carefully and use robots where it's too dangerous to go themselves. They haven't encountered any people living underground, but have found foxes, porcupines and snakes.
They have also found risque graffiti underneath the San Cosimato convent near Rome, where the Claudio and Marcio aqueducts run parallel. The words date to 18th century monks, who were jealously accusing one another of having liaisons with other monks.
Apart from the aqueducts, the team has been called on to map chambers deep beneath Palatine Hill in Rome and to explore the tunnels under the Baths of Caracalla there and at Hadrian's Villa near Tivoli. Beneath the heart of Rome, Placidi's volunteers explored the Cloaca Maxima, the massive Roman sewer that still serves the city.
"It works so well people simply forgot about where exactly it runs," Placidi said.
The aqueduct exploration coincides with the gradual crumbling of many of the above-ground arched structures in the countryside around Rome.
"Roots are the problem, and many structures have trees growing on top of them," Baldi said, pointing to a large, collapsed section of Hadrian's handiwork. "That part was still standing when Ashby was here," he said.
Today, the valley, where a section of the lane heading to the aqueduct is still paved with Roman basalt, is unsupervised.
"More people come here to illegally dump rubbish than to see the aqueduct," Baldi said.
The cavers, young and old, rarely get paid for their work by the cash-strapped Italian government, even if their results are happily being collated by archaeological authorities. Placidi combines his speleology with work as a webmaster; Baldi is an unemployed car parts dealer.
Placidi predicts that will change. "Now you have amateur cavers becoming experts on archaeology, but in 20 years' time the archaeologists will be training up as cavers," he said.