For almost four centuries scholars have sought the headwaters of the Aqua Traiana, a stone channel which carried spring water down to Rome from near Lake Bracciano. Now, two British film-makers say they have beaten the archaeologists in discovering the source of the water feeding the ancient city's greatest aqueducts. While researching films on Roman aqueducts, Mike and Ted O'Neill got access last year to a series of reservoirs and tunnels below a long-abandoned medieval chapel near the town of Bracciano. Local people believed the complex was created in late Renaissance times. But Ted O'Neill, 37, who has made a study of ancient hydraulic engineering, said he was struck by the criss-cross patterned wall facing of the tunnels. "It is known as opus reticulatum. And opus reticulatum says 'I am [ancient] Roman'," he said. The London-born brothers took Italy's leading authority on classical aqueducts to the site. Prof Lorenzo Quilici, of Bologna University, said yesterday: "It is a truly exceptional discovery. There is no doubt that the construction techniques used are ancient Roman." Quilici said the abandoned chapel, known as the Madonna of the Flower, was originally a nymphaeum, a place dedicated to the water spirits of classical mythology. "On either side it widens into two basins that are roofed with quite extraordinary vaults, still decorated with Egyptian blue [calcium copper silicate] paint," said Quilici. Prof Allan Ceen, of Pennsylvania State University, said of the site: "It is so richly decorated the emperor almost certainly came here for the inauguration of the aqueduct." That was in AD 109, under the emperor Trajan, 19 centuries before its rediscovery. To celebrate the event a fountain was built on Janiculum hill where the aqueduct entered the city. A coin was minted showing a god atop tumbling water, reclining under a broad arch. It had been assumed the arch belonged to the fountain. But the O'Neill brothers believe the coin depicts the nymphaeum, a theory Quilici thinks should be taken seriously. Not the least important aspect of the complex is that the water, which came from an aquifer, seeped into the reservoirs on either side of the nymphaeum through bricks laid with gaps between them. "It was a filter," said Quilici. The original Aqua Traiana, one of Rome's 11 great aqueducts, snaked around Lake Bracciano collecting water from other springs before heading south. At the entry point to the ancient city the aqueduct fell steeply down Janiculum hill, the water powering a chain of flour mills. The aqueduct continued to be used into the 20th century. But under Pope Paul V (1605-21), the headwaters were dispensed with and the water supply came from Lake Bracciano instead. The water from the aquifer under the Madonna of the Flower chapel was diverted to Bracciano, and today still supplies the town. The complex is now part of a pig farm whose owners use the old nymphaeum as a rubbish tip. Tree roots are pushing through the Egyptian blue decoration. "The chapel and aqueduct are in danger of crumbling. They desperately need to be restored," said Ted O'Neill.