[XVI.] Acheron was the fabled river of the lower world. Virgil probably had in his mind the real Acherusia palus, a gloomy marsh near Naples.
[XVIII.] There was a volcanic lake near Cumae called Avernus, whose waters gave out sulphureous vapours. It was connected by tradition with the lower world. Orpheus, the mythical poet, so charmed the gods of the nether world by his harp-playing, that he was allowed to take back to the upper world his dead wife Eurydice. Castor was mortal, but his brother Pollux was immortal; so when the former was slain in fight Pollux obtained from Jupiter permission that each should spend half their time in heaven, half in Hades. Theseus descended into Hades in order to carry off Proserpine. He was kept a prisoner there until he was rescued by Hercules (Alcides), who came down to carry off Cerberus, the three-headed dog who guarded the entrance (see [stanza lvi.]).
[XXXII.] Virgil alludes to the promontory of Misenum on the north side of the bay of Naples. The legend is a purely local one. There is no mention of Misenus in Homer.
[XXXIII.] 'Aornos' is a Greek word—'where no bird can come.'
[XXXV.] 'The Furies' mother and her sister' were Night and Earth.