Avernus
2 of the 7 encyclopedias on this shelf carry an entry for Avernus. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
or AVERNO, a lake in the neighborhood of Naples, about 2½ miles N. W. of Puzzuoli, and near the coast of Baiæ, the waters of which were so unwholesome and putrid that no birds ever visited its banks. The ancients made it the entrance of hell, by which Ulysses and Æneas descended into the lower regions. In the time of Vergil, a communication between it and the neighboring Lucrine Lake was made by Agrippa; but in 1538, the latter was filled by a volcanic eruption, when Monte Nuovo rose in its place, rendering the Averno again a separate lake. On its banks, instead of pestilential marshes, are now beautiful gardens and vineyards. Here was placed the grave of Hecate, and the grotto of the Cumean sibyl is still to be seen here.
Aiton's Encyclopedia (1910)
a-ver'nus, a small lake in Campania, Italy, not far from Naples. It was believed by the ancients to be the entrance to the infernal regions. The lake fills the crater of an extinct volcano. It is very deep, and was formerly surrounded by immense and gloomy forests. Owing to poisonous vapors arising from its waters, no animal life was found on its shores, and birds attempting to fly across it were killed. Avernus is situated, moreover, in the volcanic region near Vesuvius, where the earth is frequently shaken by imprisoned vapors. Strange rumbling noises are heard. It is not strange that the fables concerning the world of shades should have centered here. The lake, it is said, has lost its wild and desolate appearance in modern times. 2026 Editor's Note: Lake Avernus gave Virgil one of the most quoted lines in Latin — 'facilis descensus Averno,' the descent to the underworld is easy; it's the return that's hard. The real crater lake still lies quietly near Naples, its deadly vapors long gone, ringed now by vineyards rather than gloomy forests. (Ed: BR 2026-07-03)