Averno (anc. At emus), a lake in Italy, about 8 m. W. of Naples, and near the ruins of ancient Cumae. It lies in the crater of an extinct volcano, and, though less than 2 m. in circumference, is of great depth. It has no natural outlet, but an artificial passage for its waters into the gulf of Baire was made by Agrippa, who also connected it with the Lu-crine lake. This latter passage was closed by a volcanic convulsion which in 1538 cast up a hill of considerable height in the place of the latter lake. No attempt has been made to reopen the communication thus obstructed; and as the subterranean tunnel which connected Averno directly with the sea has also been blocked up, the lake is again without an outlet. In ancient times, Avernus, with the wild and gloomy scenery about it, the pestilent vapors rising from its volcanic shores, and the prevailing belief in its unfathomable depth, was reputed the entrance to Hades, and was made sacred to Proserpine. By this path Ulysses, according to the legend, visited the ghosts of the dead, and here was also a famous oracle. The lake retains few of its ancient characteristics; the dense woods which anciently covered its banks were cut down before the time of Strabo, .and the volcanic phenomena appear to have entirely ceased.
The ruins of a Roman edifice, probably a bath, are on the S. E. border of the lake.