Grotta di Cocceio, also known as the Cocceius Tunnel, is a straight-line subterranean gallery nearly a kilometre in length connecting Lake Avernus with Cumae north of Naples, Italy. It was burrowed clean through the tuff 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 entrance.
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, which, at the time, still bore many rich items.
The Grotta was heavily damaged during World War II and is no longer open to the public.