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.