The "Grotta di Matromània", which is half hidden amid the grand scenery of the rocks dropping sheer down the extreme south-eastern side of the island, doubtlessly retains the memory of the deity to which it was dedicated in its name. But the cult of Mitra was certainly brought to Capri later than the Julian-Claudian age; and it is thus obvious to think, rather than of Mitra, of the "Magna Mater" whose Corybantic cult is attested in the Sorrento peninsula during the reign of Domitianus by the poet Statius. The savage beauty of the site among woods and rocks should have rendered it particularly suited to the Orgiastic rites of the goddess Cybele.
Whether this place was dedicated to the "Magna Mater" or not, the Grotto has the appearance of an imposing natural cave transformed into a luxurious nymphaeum in which was collected the dripping water which filtered through the rocks above, formerly more abundantly than now, into a small cave at the bottom of the grotto. The irregularly shaped Grotto was consolidated and rendered more regular with massive masonry structures by the Romans, so as to assume the shape of a rectangular apsed hall; the walls at the two sides originally supported the vaulted ceiling of the Grotto; the end being formed by two high semicircular plinths and by the natural rock wall, out of which flowed a spring of fresh water that was collected in a small hollow; this precious clear water could be reached by a row of steps.
These structures and arrangements clearly point out the character of this mysterious grotto; it was not a sanctuary, but an arrangement whereby the hidden water springs of a natural cave were adapted to the more noble and luxurious function of a nymphaeum. Its decoration cannot have been less refined than the one we have recognized in another nymphaeum, that of "Grotta dell'Arsenale": the evidence of mosaic tesserae of glass paste, incrustations imitating stalactites and mother of pearl, molluscan shells and pods which were collected in large amounts during the haphazard explorations of antiquarians and so-called archaeologists, proves that the semicircular plinths, the walls and the vault were covered here too with a flashy polychrome decoration of stuccoes and mosaics, according to the taste and the fashion of the Hellenistic nymphaea which were adopted by the Romans in their finer town houses and in their most sumptuous villas.
From "Capri. Its History and its Monuments" by Amedeo Maiuri, published by Istituto Poligrafico dello Stato.
