Grotta di Cocceio

The Grotta di Cocceio is an ancient Roman tunnel, nearly a kilometre long, burrowed through the tuff stone of Monte Grillo to connect Lake Avernus with Cumae. It was excavated between 38 and 36 BC by the architect Lucius Cocceius Auctus, at the command of Agrippa, who was then converting the lake into a military port called Portus Julius. The tunnel’s purpose was to provide a direct, sheltered route for supplies and movement, but its engineering is what impresses: a straight, dark passage cut through volcanic rock, now a quiet remnant of Roman military logistics. It is one of the earliest known road tunnels in history, and its survival speaks to the skill of its builder, whose name it still bears.

Description via source. Coordinates and heritage data from the Atika atlas.