Avernus, a deep lake in Italy, near Naples, 1½ m. in circumference, occupying the crater of an extinct volcano, at one time surrounded by a dark wood, and conceived, from its gloomy appearance, as well as from the mephitic vapours it exhaled, to be the entrance to the infernal world, and identified with it.

AVERNUS, n. The lake by which the ancients entered the infernal
regions. The fact that access to the infernal regions was obtained by
a lake is believed by the learned Marcus Ansello Scrutator to have
suggested the Christian rite of baptism by immersion. This, however,
has been shown by Lactantius to be an error.

Facilis descensus Averni,
The poet remarks; and the sense
Of it is that when down-hill I turn I
Will get more of punches than pence.

Jehal Dai Lupe