The Grotta di Nettuno, Villa Gregoriana, Tivoli.

The micromosaic here examined shows a look inside the so-called Neptune's cave in Tivoli: a large cave in the rocky wall below the town where the waters of the Anene flowed after their fall in the "Big Waterfall". The Grotta di Nettuno was one of the major attractions of Tivoli and constituted a stage of rigor for the "Grand-Tourists" of the eighteenth century.

The course of the Aniene was changed after a flood in 1826, and the waterfalls, famous in the eighteenth century, are no longer visited in their ancient form. The cave of Neptune, however, has remained unchanged. For the subject see Sante Viola, Chronicle of the various events of the Aniene River in Tivoli, Rome 1835.

Here are some pages of the "Antiquarian Journey in Rome" (1819) where Antonio Nibby describes the Neptune Caves.

“We will begin this journey from going down to the cave of Neptune located under the cliff, above which are the temple of Vesta, and the alleged temple of the Sibyl. We descend to this cave by a convenient, multi-storey road, cut in the boulder of the cliff, and made comfortable, and safe by the Count Miollis General of the French troops stationed in the Roman States, in the year 1808. Before, the road was not only more incommoda, but often there were fatal examples of people who, attracted by too imprudent curiosity, had fallen into that abyss.

This road is shaded by vines, and lined with flowers and shrubs, which together with the natural beauty of the site make it very pleasant and delightful. We reach the end in sight of the cave, which the waters themselves have dug to find an exit. Inside this cave they can be seen together with horror, and with pleasure rushing precipitously the waters of the Anene which have become white as the foam, which form a thousand games, and dissolve in such minute sprays, which look like fog. Opposite you see another equally magnificent drop of water, which comes like snow to rush into this same depth.

About the name of Grotta di Nettuno, this is totally modern; it is not certain if the ancients knew this cave; but nevertheless I will be allowed to assert, that the Domus Albuneae resonantis of Horace can suit this cave, since rather than imagine with this expression appointed by Horace the temple of the Sibyl Albunea, or Tiburtina, which he probably did not know, is very more likely, what did he mean by any cave with a fall of water, in which the nymph of this same name lived?

After having admired this magnificent spectacle of nature, going up a little, and deviating to the right, you go down a less comfortable, and less safe, but equally pleasant road, to another cave still deeper, to which the modern ones, for pleasure , which is felt in going there, and because of the danger with which it is accompanied, they gave the name of the Grotto of the Sirens. There the reunited waters of the Aniene, after the first great fall, and after that of the cave of Neptune, go to get lost in a chasm and then reappear in the delightful valley subjected to Tivoli, where the Anene winds through the gardens, and to the vegetables. "