From Wikipedia: "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 a????? (originally a??????) 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."