The Archbasilica of St. John Lateran doesn’t quite look its age. The basilica, where the Pope presides in his role as Archbishop of Rome, was already ancient when it was rebuilt in the 1650s. Its walls still hold some of the original material used to build the cathedral under Emperor Constantine in 312 CE. And beneath the modern church lies the original Roman foundation. Excavations since the 1700s have opened up a network of dark, cramped spaces called scavi beneath the four-hectare site of the cathedral.

Centuries of Roman history lie buried in the darkness in layers stretching down to 8.5 meters (27.89 feet) below the modern floor of the cathedral, and the subterranean archaeological sites are like a honeycomb through the city’s Caelian Hill. Now, using a combination of laser scanning and ground-penetrating radar, archeologists have made a complete map of the site.

Basilica, now in 3D

Much of what’s in the scavi has been excavated and studied before, but Lateran Project co-director Ian Haynes and his colleagues say their work is the first detailed survey of the entire underground complex of ruins. They started mapping the exposed sites in the scavi with laser scans in 2012. That work, now completed, allows them to create a digital map of everything that’s currently visible thanks to the old excavations.

Still, a fair number of areas haven't been excavated. To better understand what’s there, the archaeologists are surveying the area with ground penetrating radar to map layers of buried structures.

So far, that data has helped create a 3D digital reconstruction of what the basilica would have looked like in the 4th century. And Haynes and his colleagues are also trying to understand what it would have sounded like. Using the laser scans and information from earlier excavations, they created a simple 3D model to reconstruct the acoustics of the original cathedral.

In 312 CE, the Caelian Hill overlooked most of the city of Rome. From that vantage point, a fort called the Castra Nova (New Fort, although by then it was a century old), the headquarters of the Imperial Horse Guard, dominated the Roman skyline. At the time, Rome’s four emperors—a ruling group called the Tetrarchy—were fighting among themselves for greater shares of imperial power, and the Imperial Horse Guard staunchly supported Emperor Maxentius.

At the Battle of Milvian Bridge in October of 312, Emperor Constantine defeated Maxentius. Legend claims that the victory came after a vision urged Constantine to paint a Christian symbol on his soldiers’ shields. In the weeks after the battle, Constantine ordered the New Fort razed and gave the land to the Church.

Shifting power, shifting landscape

For Constantine, it must have been like killing two birds with one stone; he stripped power and wealth away from his enemies’ supporters and gave it to his new religious allies. That moment marked a general shift of power in Rome toward religious institutions instead of the Roman military, while Constantine continued consolidating the Tetrarchy’s power in his own hands.

Sections of the former fort, including the headquarters building and barracks, still lie beneath the modern cathedral, and archaeologists are studying how Roman architects worked elements of the fort’s buildings into the construction of the new basilica.

Beneath the fort lie the remains of the palatial home of one of Rome’s elite families. A hundred years before Constantine, Emperor Septimius Severus had destroyed a whole district of such homes to build his New Fort—another shift in power in the empire. Excavations have revealed intact frescoes from the walls of the ancient Roman home, along with the remains of a bath complex, an oratory, and a market.

“To access some of the spaces, we worked with a group called Roma Sotteranea who specialize in working on buried sites and use exactly the same equipment and techniques as potholers,” said Haynes in Current World Archaeology. “In some places, it was necessary to rotate the teams on a half hourly basis because otherwise it just becomes stifling.”