Avernus

Avernus was an ancient name for a volcanic crater near Cumae (Cuma), Italy, in the Region of Campania west of Naples. Part of the Phlegraean Fields of volcanoes, Avernus is approximately 3.2 kilometres (2.0 mi) in circumference. Within the crater is Lake Avernus (Lago d'Averno).

Avernus was believed to be the entrance to the underworld, and is portrayed as such in the Aeneid of Virgil. The name comes from the Greek word aornos (ἄορνος < a- ‘no’ + ornis ‘bird’), meaning "without birds", because according to tradition, all birds flying over the lake were destined to fall dead. This was likely due to the toxic fumes that mouths of the crater gave off into the atmosphere. In later times, the word was simply an alternate name for the underworld.

On the shores of the lake is the grotto of the Cumaean Sybil and the entrance to a long tunnel (Grotta di Cocceio, ca. 800 metres (2,600 ft)) leading toward Cumae, where her sanctuary was located. There are also the remains of temples to Apollo and Jupiter. During the civil war between Octavian and Antony, Agrippa tried to turn the lake into a military port, the Portus Julius. A waterway was dug from Lake Lucrino to Avernus to this end. The port's remains may still be seen under the lake's surface.

...
Wikipedia