About 2000 years after they were hacked out of solid rock by Roman engineers, the aqueducts that brought fresh water to ancient Rome are being explored anew with 21st-century technology.

Archaeologists with specialist caving, abseiling and potholing experience are using lasers, remote-controlled robots and 3D scanners to map the dozen aqueducts that were built over centuries.

They are working from maps drawn by Thomas Ashby, a British topographer and archaeologist, who explored the hidden tunnels before and after World War 1. Almost a century later, the new breed of explorers is measuring the aqueducts with precision he could have only dreamt of.

They are using 3D scanners mounted on tripods to produce accurate images of the inside of the tunnels, which are lined with Roman-era concrete so smooth and unblemished, it looks as if it had been applied a few years ago.

Laser "rangefinders" enable the archaeologists to measure the size, direction and elevation of the tunnels east of Rome.

The mapping effort is being led by amateur archaeologists with caving expertise from a group called Sotterranei di Roma (Underground Rome).

"The Romans were incredible engineers," said Alfonso Diaz Boj, 52, a Spanish member of the group.

"To slow down the flow of the water they built curved sections. Teams of diggers would start from opposite directions and they would meet in the middle."

The excavation was carried out by specialist engineers, he said, but the soil would have been removed by slaves.

"Ninety percent of these aqueducts are tunnels - it was much easier to burrow underground than to build channels supported on pylons above ground," he said.

Pick marks left by their tools were clearly visible on the roof of the aqueduct being explored, known as the "Aqua Claudio".

Construction began during the reign of the Emperor Caligula in 38AD and finished under that of the Emperor Claudius, 14 years later. It ran for almost 80km , from the mountains beyond the modern towns of Tivoli and Frascati.

Other aqueducts extended more than 95km from the city. The water arrived at one of the ancient city's gates, the Porta Maggiore.

Others in the group are going underground in the city itself, exploring the cisterns, drains and tunnels beneath the Roman Forum with the help of a remote-controlled robot named Lucius.

The six-wheeled "archeo-robot", equipped with two powerful computers, three high-definition cameras and laser sensors, trundles along narrow passageways too small or dangerous for people.

"It's not very nice down there and there's often a build-up of gases, so robots are ideal," said Christopher Smith, the director of the British School at Rome, an archaeological research institute.

"There are miles of tunnels beneath sites like the Colosseum, for instance, much of which we know little about."