Strabo Geography 5.4.5

[5] Near to Cumae is the promontory of Misenum, ¹ and between them is the Acherusian Lake, ² which is a muddy estuary of the sea. Having doubled Misenum, you come to a harbour at the very foot of the promontory. After this the shore runs inland, forming a deeply indented bay, on which are Baïae and the hot springs, much used, both as a fashionable watering-place, and for the cure of diseases. Contiguous to Baïae is the Lucrine Lake, ³ and within this the Lake Avernus, ⁴ which converts into a peninsula the land stretching from the maritime district, situated between it and Cumae, as far as Cape Misenum, for there is only an isthmus of a few stadia, across which a subterraneous road is cut [from the head of the gulf of Avernus] to Cumae and the sea [shore] on which it stands. Former writers, mingling fable with history, have applied to Avernus the expressions of Homer in his Invocation of Departed Spirits, ⁵ and relate that here formerly was an oracle of the dead, ⁶ and that it was to this place that Ulysses came. However, this gulf of Avernus is deep even near the shore, with an excellent entrance, and is both as to its size and nature a harbour; but it is not used, on account of the Lucrine Gulf which lies before it, and is both large and somewhat shallow. The Avernus is surrounded with steep hills which encompass the whole of it, with the exception of the entrance. These hills, now so beautifully cultivated were formerly covered with wild forests, gigantic and impenetrable, which overshadowed the gulf, imparting a feeling of superstitious awe. The inhabitants affirm that birds, flying over the lake, fall into the water, ⁷ being stifled by the vapours rising from it, a phenomenon of all Plutonian ⁸ localities. They believed, in fact, that this place was a Plutonium, around which the Kimmerians used to dwell, and those who sailed into the place made sacrifice and propitiatory offerings to the infernal deities, as they were instructed by the priests who ministered at the place. There is here a spring of water near to the sea fit for drinking, from which, however, every one abstained, as they supposed it to be water from the Styx: [they thought likewise] that the oracle of the dead was situated some where here; and the hot springs near to the Acherusian Lake indicated the proximity of Pyriphlegethon. Ephorus, peopling this place with Kimmerii, tells us that they dwell in under-ground habitations, named by them Argillae, and that these communicate with one another by means of certain subterranean passages; and that they conduct strangers through them to the oracle, which is built far below the surface of the earth. They live on the mines together with the profits accruing from the oracle, and grants made to them by the king [of the country]. It was a traditional custom for the servants of the oracle never to behold the sun, and only to quit their caverns at night. It was on this account that the poet said,

On them the Sun

Deigns not to look with his beam-darting eye. ⁹

Odys. xi. 15. At last, however, these men were exterminated by one of the kings, the oracle having deceived him; but [adds Ephorus] the oracle is still in existence, though removed to another place. Such were the myths related by our ancestors. But now that the wood surrounding the Avernus has been cut down by Agrippa, the lands built upon, and a subterranean passage cut from Avernus to Cumae, all these appear fables. Perhaps ¹⁰ Cocceius, who made this subterranean passage, ¹¹ wished to follow the practice of the Kimmerians we have already described, or fancied that it was natural to this place that its roads should be made under-ground.