Grotta di Cocceio Grotta di Cocceio , also known as the Cocceius Tunnel , is a straight-line subterranean gallery nearly a kilometer in length connecting Lake Avernus with Cumae north of Naples , Italy . It was burrowed clean through the tufa stone of Monte Grillo from 38-36 BCE. The tunnel was effected by the architect Lucius Cocceius Auctus at the wishes of Agrippa who was in the process of converting the Lake into a military port , the Portus Julius . The Avernus side of the passage was decorated with a colonnade and had many statues in niches hewn into the tufa stone walls of the entance . It is also known as the Grotta della Pace in reference to a Spanish captain , Pietro de Pace , who made use of the tunnel in 1508-1509 to plunder the ruins of Cumae as yet still bearing rich items . The Grotta was heavily damaged during World War II and is no longer open to the public . Categories : Tunnels in Italy 