Lake Avernus (or Lago d'Averno in Italian) is a volcanic crater lake located in the Avernus crater in the Campania region of southern Italy, around 4 km (2.5 mi) northwest of Pozzuoli. It is near the volcanic field known as the Campi Flegrei and comprises part of the wider Campanian volcanic arc. The lake is roughly circular, measuring 2 km (1.2 mi) in circumference and 60 m (213 ft) deep.
Avernus was of major importance to the Romans, who considered it to be the entrance to Hades. It gained its name from the Greek word αορνος (originally αϝορνος) meaning "birdless", referring to the belief that birds flying over the lake would drop dead from the poisonous fumes that it emitted. The name Avernus was often used by Roman writers as a synonym for the underworld. In Virgil's Aeneid, Aeneas descends to the underworld through a cave near the lake. In Hyginus' Fabulae, Odysseus also goes to the lower world from this spot where he meets Elpenor, his comrade who went missing at Circe's place.
It is unclear whether the lake actually was as deadly as its reputation held it to be – it certainly holds no fears for birds today – but it is possible that volcanic activity could have produced deadly fumes. Volcanically active crater lakes can be extremely dangerous places, as illustrated by the tragic example of Lake Nyos in Cameroon.