Avernus, a lake of Campania, Italy, about 1&FRAC12; m. N. of Baiae. It is an old volcanic crater, nearly 2 m. in circumference, now, as in Roman times, filled with water. Its depth is 213 ft., and its height above sea-level 3&FRAC12; ft.; it has no natural outlet. In ancient times it was surrounded by dense forests, and was the centre of many legends. It was represented as the entrance by which both Odysseus and Aeneas descended to the infernal regions, and as the abode of the Cimmerii. Its Greek name, Ἄορνος, was explained to mean that no bird could fly across it. Hannibal made a pilgrimage to it in 214 B.C. Agrippa in 37 B.C. converted it into a naval harbour, the Portus Iulius; joining it to the Lacus Lucrinus by a canal, and connecting the latter with the sea, he reduced the distance to Cumae by boring a tunnel over &FRAC12; m. in length, now called Grotta della Pace, through the hill on the north-west side of Lake Avernus. After Sextus Pompeius had been subdued, the chief naval harbour was transferred to Misenum. Nero's works for his proposed canal from Baiae to the Tiber (A.D. 64) seem to have begun near Lake Avernus; indeed, according to one theory, the Grotta della Pace would be a portion of this canal.
On the east side of the lake are remains of baths, including a great octagonal hall known as the Temple of Apollo, built of brickwork, and belonging to the 1st century. The so-called Grotto of the Cumaean Sibyl, on the south side, is a rock-cut passage, ventilated by vertical apertures, possibly a part of the works connected with the naval harbour. To the south-east of the lake is the Monte Nuovo, a volcanic hill upheaved in 1538, with a deep extinct crater in the centre. To the south is the Lacus Lucrinus.
See J. Beloch, Campanien (2nd ed., Breslau, 1890), pp. 168 seq.