Avernus was an ancient name for a crater near Cumae (Cuma), Italy in the Region of Campania north of Naples. Within the crater is Lake Avernus (Lago d'Averno). It was believed to be the entrance to the underworld, and is portrayed as such in the Aeneid of Virgil. In later times, the word was simply an alternate name for the underworld. On the shores of the Lake is the grotto of the Cumaean Sybil and the entrance to a long tunnel (Grotta di Cocceio, ca. 800 meters) leading toward Cumae, where her sanctuary was located. There are also the remains of temples to Apollo and Jupiter. During the civil war between Octavian and Antony, Agrippa tried to turn the lake into a military port, the Portus Julius. A waterway was dug from Lake Lucrino to Avernus to this end. The Port's remains may still be seen under the Lake's surface.
The term Avernus (plural Averni) was also used by ancient naturalists for certain lakes, grottos, and other places which infect the air with poisonous steams or vapors. They were also called mephites. They were said to be frequent in Hungary on account of the abundance of mines therein. The Grotto dei Cani in Italy was a famous example. The most celebrated of these, however, is Lake Avernus. The fumes it emitted were represented by the ancients as of so malignant a nature, that birds could not fly over it, but fell down dead. In addition, Mephitis was the Roman goddess of noxious vapors, who protects against malaria. The adjective "mephitic" means "foul-smelling" or "malodorous".