The cave called San Giovanni is the only case in Italy and one of the three in the whole world, where the cave is overstepped by an illuminated cement road , actually practicable only by foot in order to preserve the environmental conditions. It is an extraordinary “carsic” phenomenon, this is one of the longest natural cave with its 850 metres. Its name comes from the presence of an internal chapel dedicated to the Saint. It is believed that it has been formed due to a downfall of a huge quantity of limestone of the mount Acqua . Nex to the hall in the southern part it is possible to admire an alabaster mould 15 metres high with a diameter of 6 metres and a fount that gathers Cagliari and Domusnovas waterworks. Inside the cave it is possible to see many different huge rooms with a lot of stalagmites and a few stalactites, and beautiful basin containers (considered by the experts a great Italian example).It is composed by different lateral tunnels . One of these tunnels halfway reaches out towards the west and it is considered the second branch of the cave obtaining 2000 metres of extension field. This tunnel is called “Su stampu de Pireddu” (Pireddu’s Hole) , it is formed of different pipelines joined together like the ones of the underground creek that feeds San Giovanni’s source near the southern entrance . Going out , in the northern part of the cave , it is possible to admire wonderful woods where are still present mineral deposits fallen into disuse .