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 BC. 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.