AVERNUS, lake of Campania, Italy, about 1 km. N. of Baiae. It is an old volcanic crater, nearly 2m. in circumference, now, as in Roman times, filled with water. Its depth is 213ft., and its height above sea-level 3-ft. ; it has no natural outlet. In ancient times it was surrounded by dense forests. 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 "Aopvos, was explained to mean that no bird could fly across it. Hannibal made a pilgrimage to it in 2 14 B.C. Agrippa in 37 B.C. converted it into a naval harbour, the Portus Julius; and joining it to Lacus Lucrinus by canal connecting with the sea, he reduced the distance to Cumae by boring a tunnel over im. in length, now called Grotta Pace, through the hill on the north-west side of Lake Avernus. The canal, however, soon became blocked owing to a gradual rise of the shore. 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 Pace would be a portion of this canal. On the east side of the lake are remains of baths, including a great octagonal brickwork hall known as the Temple of Apollo (1st century) . The so-called Grotto of the Cumaean Sibyl, on the south side, is a rock-cut passage, ventilated by vertical apertures, possibly part of works connected with the naval harbour.