Avernus, a-ver'nus, a small lake in Campania, Italy, not far from Naples. It was believed by the ancients to be the entrance to the infernal regions. The lake fills the crater of an extinct volcano. It is very deep, and was formerly surrounded by immense and gloomy forests. Owing to poisonous vapors arising from its waters, no animal life was found on its shores, and birds attempting to fly across it were killed. Avernus is situated, moreover, in the volcanic region near Vesuvius, where the earth is frequently shaken by imprisoned vapors. Strange rumbling noises are heard. It is not strange that the fables concerning the world of shades should have centered here. The lake, it is said, has lost its wild and desolate appearance in modern times.