Archaeologists use potholing techniques to map Roman ruins

Squeezing into claustrophobic tunnels on their stomachs, British archaeologists have mapped a hidden world of Roman ruins lying beneath the world's first cathedral.

The experts employed potholing techniques and laser instruments as they squirmed their way through shafts and chambers 9m beneath the 17th-century Basilica of St John Lateran in Rome.

The present-day basilica stands on the ruins of a 4th-century basilica founded by the Emperor Constantine, who adopted Christianity as the official religion of the Roman Empire.

The basilica was in turn built on top of a huge barracks that housed a detachment of elite Roman imperial cavalry.

Beneath the barracks, archaeologists explored yet another layer of Roman ruins - sumptuous villas decorated with extravagant frescoes, some of them marked with graffiti left by Roman soldiers and military engineers. The legionaries recorded their names and units as they dismantled the villas, carting off marble and other materials to be re-used in the construction of the barracks.

Among the graffiti was a sketch of an ostrich hunt which probably took place in the nearby Colosseum, where gladiators were matched against wild animals as well as each other.

One of the unfortunate birds appears to have had its head chopped off with a sword. The emperor Commodus, a debauched megalomaniac, is reputed to have fought in the Colosseum and to have pitted himself against ostriches and other animals. The graffiti may be a depiction of him in action.

"Some of the areas we explored were very difficult to access - dark passageways where you're working on your stomach," said Professor Ian Haynes, the co-director of the five-year research effort, The Lateran Project.

"Getting the laser scanners into position in such narrow, confined spaces is complicated. In some places, it was necessary to rotate the teams on a half-hourly basis because otherwise it just becomes stifling."

Using laser instruments and ground-penetrating radar, the team produced 3D digital reconstructions of what the early basilica and the cavalry barracks would have looked like.

The barracks, known as the Castra Nova or New Fort, accommodated hundreds of cavalry soldiers and their horses. They were built under the rule of the emperor Septimius Severus.

An identical barracks was located a short distance away - a deliberate arrangement by which the imperial cavalry were split in two so they could not plot against the emperor as one unit.

"It's a reflection of imperial security paranoia," said Haynes, from Newcastle University, who worked on the project with Italian and Dutch archeologists and with the support of the British School at Rome.