Today the tomb and relics of St. Sebastian are housed in the ground-level basilica, but his original resting place was in the catacombs beneath it. Sebastian was a senior Milanese soldier in the Roman army who converted to Christianity and was martyred in the first decade of the 4th century, during Emperor Diocletian’s persecutions, which were especially brutal. From the reign of Valerian to that of Constantine, the bodies of Sts. Peter and Paul were also hidden in the catacombs, which were dug from tufa, a soft volcanic rock that hardens on exposure to the air.
The underground passages, if stretched out, would reach a length of 11km (6 3/4 miles). In the tunnels and mausoleums are mosaics and graffiti, along with many other pagan and Christian objects, as well as 4 Roman tombs with their frescoes and stucco fairly intact. They were found in 1922 after being buried for almost 2,000 years.