Before descending to the underground complex, about 3 meters (9.8 feet) below, the O'Neills recruited Lorenzo Quilici, a leading authority on Roman hydro-engineering from Bologna University, and Allan Ceen, professor of history of architecture at Pennsylvania State University.
Quilici confirmed that the building was Roman, rather than medieval, as had long been believed.
"It's all Roman. The brickwork and waterproof hydraulic cement lining the tunnels is absolutely characteristic of the Trajanic age," Quilici said.
Beyond the subterranean chamber, a 125-meter-long (410-foot-long) gallery led to the beginning of the aqueduct. But what struck the researchers was the chamber's decorations, made with a rare and costly type of paint known as Egyptian blue (calcium copper silicate).
"This was an extraordinary monument, a vaulted, three-chambered semicircular nymphaeum (a monument consecrated to the nymphs in ancient Greece and Rome). At the center there was a small temple dedicated the the spring god, while on both sides there were two basins," Quilici said.
Roofed with quite extraordinary vaults, still decorated with Egyptian blue, the basins filtered the spring water through bricks laid with gaps between them.
"The basins had two functions: they collected the waters for the aqueduct and provided quite beautiful scenery," Quilici said.
According to the researchers, the richly decorated vaulted ceilings suggest that Trajan (the 13th Roman emperor) almost certainly came there for the aqueduct's inauguration.
Indeed, the emperor may have been in that area on June 24, 109 A.D., according to historical records.
"By coincidence we first explored the aqueduct on June 24, 2009, exactly 1,900 years later," O'Neill pointed out.
Trajan commemorated the opening of the aqueduct by minting a Roman coin and building a fountain on Janiculum Hill, right where the waters entered the city.
The coin shows a river god atop flowing waters, reclining in what looks like a grotto or a tunnel.
According to Rabun Taylor, professor of classics at the University of Texas at Austin, this is an unique finding.
"This is a discovery of almost unprecedented importance in the long history of aqueduct studies," Taylor, who has published widely on the architecture and hydraulics of the city, said in a statement.
In use until the Renaissance, the aqueduct was rebuilt by Pope Paul V between 1605 and 1615 and renamed Aqua Paola after him.
Taking the water directly from Lake Bracciano, and not from the nearby springs as in Roman times, the papal aqueduct still brings water to Rome, culminating almost on the same spot on Janiculum hill.
"The water however, it is not as fresh and pure as in Trajan's times. It is actually quite nasty," O'Neill said.
The nymphaeum suffered an even worst fate. Located inside a pig farm, it is used today as a rubbish dump. Moreover, fig roots are pushing through the valuted ceiling.
"The site is crumbling and could totally disintegrated in 20 years. It desperately needs to be restored," said O'Neill.
http://news.discovery.com/archaeology/roman-aqueduct-emperor.html