Averno, or Tripergola (classic Avernus), a small lake in Campania, Italy, about 10 miles W. of Naples, at the head of the Bay of Baiae. It has a circumference of about one and a-half miles, and probably occupies the hollow of a crater, for its waters exhaled such mephitic vapours that no birds, according to ancient story, could fly over them, and the name was supposed to be derived from the Greek aornos, birdless. Agrippa connected it by a channel with the Lucrine Lake, but in 1538 this latter was filled up by a volcanic eruption. In classic mythology Avernus was looked upon as the entrance to the infernal regions. Recently the banks have been drained and laid out in charming gardens, a channel connects the lake with the sea, and the surrounding district is cultivated. The grotto of the Cumaean Sybil is still shown here.