We are now at the southern entrance of San Giovanni’s cave. We can refer to it as a true hydro-geologic “tunnel” inside Mount Acqua, which represents a monumental access to the valleys behind. The Tunnel is made of the main cave “gallery”, with a system of inverted faults with main directions N-S and NE – SW, completed by a series of lengthy lateral ramifications. Some of them, completely flooded, are what we call the active part of the cave system.

The Cave “Is not always been and always will be”. What we see today is the results of evolutionary mechanisms of chemical and physical origins, trigged by the calcareous nature of the rocks in this territory. The limestone itself do not solve, but when rainy waters meet the last layers of the atmosphere, they pick up carbon dioxide. Once reached the ground, waters may pass through it picking even more CO2, which makes them acidic. These acidic waters are able to break the limestone, through a process that transforms calcium carbonate into calcium bicarbonate. The latter one can dissolve, so that acidic waters fracture the limestone near its cracks. As time goes on, these fractures get wider, to reach the results that we can admire today. This process just described is called “Karst”. The underground drainage system, together with water flowing through the region in the shape of rivers, add up a physical action. This influx of running waters, with sediments transported by the speed and force of the flow, creates an even more erosive power in the evolution of karst systems.

Karsts operates on both ways. The acidic action of the water do not last forever and, when the bicarbonate is saturated (In other terms when the water is no longer able to solve) the karst process swap direction. The calcium carbonate from the cave ceiling precipitates during very slow mechanisms of deposition, slowly building the so-called speleothems, or more commonly known cave formations. They can be stalagmites, stalactites and flowstones, according to the different mechanisms of deposition. In the San Giovanni’s cave, we also find large and spectacular rimstone (gours) formations, the largest in Italy, massive flowstones and stalagmites. Fewer are the stalactites.

Human activity, since ancient times, is still visible: we stand in front of what once was a prehistoric wall closing the cave entrance. Similar remains are present on the other terminal, at the north exit. The four meters thick wall, according to bibliography, once closed the south entrance of the cave. Unfortunately, we do not have a photographic evidence of it, but a drawing published in 1826 in the book “Voyage en Sardaigne” still survives, realized by Alberto La Marmora. We have another reference coming from a Priest, Angius, who describes the wall in “Dizionario geografico storico statistico commerciale degli Stati di S. M. il Re di Sardegna” written by Goffredo Casalis in 1843. From both authors the fortified walls are proof of inhabitation of the natural cave during Neolithic times.

More Human activity modified the environment during the industrial revolution. The original route of the natural gallery will be heavily changed during the second half of the 1800, with the begging of the construction of the road. This is a sort of causeway over the riverbed, going through the whole length of the cave. Construction started in 1866, when Count Pietro Beltrami was appointed in charge of timber cutting, succeeded by Sir Pietro Semenza and the businessman Bernardo
Fabbricotti, for the exploitation of the secular forests behind Mount Acqua. The road would facilitate timber and coal transportation towards the valley. The movement of mineral production from the large mines operating in Sa Ducchessa and Reigraxius, sites lying behind the cave, soon replaced its original purpose. The road was still owned by Semenza and Fabbricotti and the passage was allowed only towards an upfront payment. (Fee based on weight transported). This makes the road one of the first toll roads of the new Kingdom of Italy.

This feature makes San Giovanni’s cave one of the only four caves in the world completely accessible by traffic vehicles, and the longest one of all with its 850 meters. The other three are the Mas d’Azil cave in France, with 450 meters of lengths, the “Cueva Dell’Agua” s cave in the Spanish region of the Asturias with 300 meters and the Jenolan cave in Australia stretching for 150 meters. This feature makes the cave one of the few caves in the world enjoyable in wheelchair.