Lake Averno is a volcanic lake

The Grotta di Cocceio (Cocceius' Tunnel) is an ancient Roman tunnel nearly a kilometre in length connecting Lake Avernus with Cumae and dating from 38-36 BC. It was burrowed through the tuff stone of Monte Grillo by the architect Lucius Cocceius Auctus at the command of Agrippa who was in the process of converting the Lake into a military port, the Portus Julius.

Lake Avernus (Italian: Lago d'Averno) is a volcanic crater lake located in the Avernus crater in the Campania region of southern Italy, around 4 km (2.5 mi) west of Pozzuoli. It is near the volcanic field known as the Phlegraean Fields (Campi Flegrei) and comprises part of the wider Campanian volcanic arc.