LECCE, Italy — All Luciano Faggiano wanted when he purchased the seemingly unremarkable building at 56 Via Ascanio Grandi was to open a trattoria. He could run the trattoria on the ground floor and live upstairs with his wife and youngest son. The only problem was the toilet. Sewage kept backing up. So Mr. Faggiano enlisted his two older sons to help him dig a trench and investigate the underground sewage pipe. He predicted the job would take about a week, and soon after he would open his trattoria. If only. “We found underground corridors and other rooms, so we...