It is the second most important cave after Neptune, and is located on the eastern side of Capo Caccia. The scenic and huge entrance is at an altitude of around 80 metres; the entire cave is very interesting from a geological, naturalistic and archaeological standpoint, since in the prehistoric age —it is estimated at least 7000 years ago—the cave was already populated by hominids looking for a shelter. On the walls, graffiti and undisputed signs of a human presence are still recognizable; drawings that many still consider shrouded in mystery, and which remain without explication. The natural light which penetrates into the wide entrance hall, allows for the growth of green mosses and lichens on the rocks. This is the feature that gave name to this place.
Imposing stalagmites, which are at least twenty metres high, are clearly visible, and continuing along towards the end of the cave, you reach the underwater part of it, where cave-divers discovered human remains, traces of ancient graves and even Neolithic vases.
The Grotta Verde (the Green Cave) was renamed as “cave of the altar”, because sailors built a small altar inside it dedicated to Saint Erasmo, their patron, back in the Fifteenth century.