The Agnano basin is the oldest crater from the third eruptive period of the Flegrean Fields (8,000-3,900 years ago). Collapsed on the western and northern sides because of later formations of the Solfatara craters and of the Astroni, the basin terminates on the southwest in Mt. Spina and on the east in Mt. Sant'Angelo di Napoli. The circumference of the basin is 6.5 km.
Since antiquity this site has been most commonly sought after because of its therapeutic value-that is, the presence of secondary volcanic phenomena (thermal springs, fumaroles and solfatare-i.e. sulfur vents). On the slopes of Mt. Spina you can see the ruins of Roman thermal baths dated to the 1st-2nd century AD. They consist of an apodyterium, a frigidarium, warm rooms (tepidaria, calidaria and laconica) as well as a series of interconnected cisterns. The complex was a way-station for those on the road between Neapolis and Puteoli; it was fed by the Serino aqueduct of Augustus, using, as well, the on-site underground hot-water springs.
Until 1870 the basin hosted a lake (artificially emptied and dried up in that year) that had appeared around the eleventh century following seismic phenomena known as 'bradisisms'. The basin was fed by 75 thermo-mineral springs reaching temperatures of up to 75 degrees C. Alphonse I transferred to the lake the linen production facility that had previously been in a swampy area beyond the Magdalene Bridge (east of Naples); the maceration (or soaking, also retting) of the hemp involved in linen production actually fouled the waters of the lake making them all the more unhealthy and even producing conditions conducive to malaria. The foul air from the lake was enough to force monks at the nearby Camaldoli monastery to move out in the summer months and relocate to smaller premises on the eastern slope leading to la Pigna, a site that became known as Camaldolilli, built especially to take in the monks who had fallen ill. In 1789 a type of quarantine line was set up at what is today via Giulio; you can still see the inscribed stone marker in the wall of the Silio Italico school: "By order of Ferdinand IV, King of Naples and Sicily, carts and animals returning from the linen works at Lake Agnano must stop here."
This apocalyptic setting did not stop the many foreign travellers who over the centuries visited the area attracted in particular by a curious affair occurring in an artificial cave on the banks of the lake-the so-called "Grotto of the Dog". For a few pennies you could take in a barbaric spectacle: a small dog would be placed snout down on the floor of the cave, and within a few seconds would shows clear signs of asphyxiation and could be revived only by being taken out into the open air quickly and/or being tossing into the lake. The "resurrection", however, was just the beginning because then the scene would be repeated on the same animal; many animals thus perished in that death chamber just for the amusement of humans! (fig. 1)
Physician Pasquale Panvini, on the other hand, decided to test on himself the effects of the terrible "mofeta" (carbon dioxide issuing from a fissure in the ground), a gas that because of its weight hugged the bottom of the cave. "I realized that the dog was not immediately affected by the gas, as some had said, so I wanted to see what it was like. I got down in the middle of the cave with my face touching the ground and breathed the gas for ten seconds. I took nine complete breaths before I felt a sense of discomfort: my eyes and nose itched slightly, and I felt a prickly sensation in my legs, arms and face. Finally, I grew short of breath; it became so difficult to breathe that I realized I could no longer continue with impunity..." (from Il forestiere alle antichità e curiosità naturali di Pozzuoli, 1818 [A Foreigner Among the Antiquities and Natural Curiosities of Pozzuoli]).
An interesting experiment was conducted in the second half of the 1700s by Simone Stratico, professor of Mathematics, Nautical Theory and Experimental Physics at the University of Padua. The results were reported in a manuscript drawn up between 1783 and 1784 and erroneously attributed to Volta: "I made a number of observations in that grotto, showing, among other things, that a magnetic needle lost its polarity; but that wasn't from the gas, since I had also tried using artificially produced gas. It's clear that the phenomenon was due to some vein of iron hidden in the walls of the grotto."
But who dug the cave and why? What was hidden in the depths that kept visitors from setting foot in it for 2000 years?
In 2001 speleologist Rosario Varriale from the Center for Speleological Research of Naples, outfitted with appropriate gear (fig. 2)-and after a complicated and tiring cleaning out of the interior of the cave-entered into a 10-meter-long passage. The greatest surprise was then finding a 32 sq. meter chamber, partially blocked, with a stepped "walkway" around the sides, with traces of cocciopesto [pottery fragments mixed with lime or sand used to make plaster or mortar] at the base. High up in one corner of the chamber there must have been at one time an opening for light. The temperature within the chamber reached as high as 60 degrees C. At the end of this article you may view some of the interesting data from the work within the grotto as well as see measurements taken by Rosario Varriale and reworked by Libero Campana (fig. 3, 4 and 5).
One theory is that the grotto was hollowed out to serve primarily as a steam room or thermal bath in the 3rd-2nd century BC, at which time the infamous gas had not yet broken into the chamber or at least was somehow kept from building up within it...
Another fascinating theory connects the grotto to those buildings from the Hellenic age of the 4th-3rd century BC found in the basin after the draining and land reclamation (1870): the ancient city submerged in the depths of a lake, the waters of which had had such a harmful effect on travellers over the centuries.
These structures reveal themselves in the form of an impressive stepped wall of tufa rock, perhaps connected to the presence of a monument of some sort at the upper levels. A temple? That would explain the presence of votive statuettes found at the site. And might the Grotto of the Dog have had some religious function connected to this sacred building? Only further and more detailed studies (and above all further excavation) can solve the riddle...
One strange thing: there are blocks of tufa semi-submerged by the waters of a thermal spring (60° C) still used for therapy. On these blocks, you can still see engraved markings (fig. 6 and 7). These are the same that you find on the ancient Greek walls (4th century BC) of Neapolis to be found at Piazza Cavour (fig. 8 and 9). They are explained as the signature marks of the original Neapolitan stonemasons. Those who built the indestructible walls of Neapolis were then called upon to raise the most ancient structures of Agnano...
We thank the volunteers of the GAN (Neapolitan Archaeological Group), who worked with the agency for the Terme of Agnano and the Civic Committee of Pendio-Agnano as authorized by the Special Superintendence for Archaeology of Naples and Pompei. These volunteers recently cleared away the vegetation and refuse at the Roman baths at Agnano, making it possible to visit this archaeological site. The guided tour is free.
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