It is the only place in the Mediterranean in which the colonial madreporario can be found, from which the coral reefs originate. To the extreme western part of the bay of Porto Conte, on the promontory of Capo Caccia, the same as the Grotta di Nettuno and the underwater one of Nereo, there is another karst cave, the Grotta Verde - also known as that of the 'Altar' - with limestone cliffs dating back to 200 million years ago. On the walls, there are stalagmites and stalactites 12 metres high and covered with plant incrustations. On the floor, there is a saltwater lake that reflects the faint light, creating an evocative greenish light. The water covers various rooms, once dry and 'lived in' by prehistoric man, as is evident in the precious graffiti: the cave was frequented from the Early Neolithic period (6th millennium BC) to the Christian era as a place of worship and a burial site.