Mentioned in the Tabula Peutingeriana (a map with road itineraries from the late imperial period) and recalled not only by Strabo but also by Donatus, Seneca, Petronius, and Eusebius, the gallery is entirely excavated in tuff rock for a length of 705 meters, an original width of 4.50 meters, and a height of about 5 meters, illuminated and ventilated by two oblique light wells.
The poor visibility inside the structure already led during the Spanish viceroyalty to the creation of a lighting system consisting of lanterns supported by ropes stretched between poles; in 1806, under Giuseppe Bonaparte, two rows of lanterns were installed and kept constantly lit, while from the mid-19th century gas lanterns were used, one of which, from the end of the century, was found in recent restoration works.
Following the widening and lowering of the road level, as well as the paving carried out in several phases by Alfonso d’Aragona in 1455, Don Pedro di Toledo in 1548, Charles of Bourbon in 1748, and the Municipality of Naples in 1893, the grotto lost much of its ancient appearance.
On the sides of the entrance there are two frescoed niches: the left one with a depiction of the Madonna and Child dating back to the 14th century, the right one with the face of the Almighty of uncertain dating. Petrarch in the Itinerarium Syriacum mentions a small chapel called Santa Maria dell’Idria, built by a hermit right near the entrance to the grotto.
During the Aragonese restoration or during works carried out in the Spanish viceroyalty era, a white marble bas-relief depicting Mithras dated between the late 3rd and early 4th centuries AD was found and is preserved in the National Archaeological Museum of Naples.
Evidence related to the oriental god Mithras is known in Campania starting from the 2nd century AD, contrasting the increasingly widespread Christianity; the presence of the relief in the Crypta has led to the hypothesis that this might be a Mithraic place of worship: the mithraeum is typically identified with the spelaeum, the cosmic cave, inside which, from the oldest iconographic testimonies, the sacrifice of the bull is depicted.
It is likely that the mystery cults significantly influenced popular superstition, which has always associated something mysterious and magical with the grotto, to the point that simply crossing it unharmed was considered a true miracle.
, was also the creator of Portus Julius, the "Grotta di Cocceio" and the Roman Crypta at Cumae.){kind=link}