In Latin, as well as in English, the toponym "Avernus" is synonymous with "hell" or "underworld." It comes from the name of a volcanic crater lake in Campania, Italy. It means "birdless" in Ancient Greek (áornos), likely named so for the toxic fumes seeping from the area, which is part of the Phlegraean Fields of active volcanoes.

In ancient Roman beliefs, Lake Avernus was considered the entrance to Hades, and its name grew to be a synonym for the underworld itself. It appears in Virgil's Aeneid, in which Aeneas descends to Hades through a cave near the lake. Later, as a generic term avernus (plural: averni), the word was also used by ancient naturalists to refer to certain lakes and grottoes that exuded fatal gases, such as the Cave of Dogs.

In 37 B.C., the lake was converted into a naval base by Agrippa as Octavian prepared to engage in a civil war against Mark Antony. It was connected to the nearby Greek colony of Cumae with the Grotta di Cocceio, which is sometimes regarded as the world's first major road tunnel and remained in use for centuries until it was heavily damaged during World War II.

Today, ruins of ancient temples dedicated to Phoebus and Juno can be found on the lakeshore, as well as a dark, mysterious grotto of the Cumaean Sibyl, once open to the public but now off-limits and all but abandoned. The lake itself is a scenic site perfect for a day-trip from Naples, surrounded by farmlands and green hills, with a variety of waterfowl (it is no longer birdless!) floating and waddling around and cats basking in the warm Campanian sun.