Prof. Lorenzo Quilici in the Aqua Traiana Photo: MEON HDTV

The underground spring lies behind a concealed door beneath an abandoned 13th century church on the shores of Lake Bracciano, 35 miles north of Rome.

Exploration of the site has shown that water percolating through volcanic bedrock was collected in underground grottoes and chambers and fed into a subterranean aqueduct, the Aqua Traiana, which took it all the way to the imperial capital.

Centuries later, it provided water for the very first Vatican, after Rome began to convert to Christianity under the Emperor Constantine.

The underground complex, which is entangled with the roots of huge fig trees, was discovered by father and son documentary makers Edward and Michael O'Neill, who stumbled on it while researching the history of Rome's ancient aqueducts.

They recruited a leading authority on Roman hydro-engineering, Prof Lorenzo Quilici from Bologna University, who confirmed that the structure was Roman, rather than medieval as had long been believed.

Using long iron ladders to descend into the bowels of the sophisticated system, they found that the bricks comprising the aqueduct's walls are laid in a diamond shape known as "opus reticulatum" – a distinctive Roman style of engineering.

"A lot of the stone work bears the original Roman tool marks," Edward O'Neill said. READ MORE