Alghero, Sardinia, Italy

Grotta di Nereo

Grotta di Nereo, located beneath the towering limestone cliffs of Capo Caccia just west of Alghero, is regarded as the largest navigable submerged sea cave in the Mediterranean. The system is a labyrinth of tunnels, halls and chimneys carved by ancient karst processes, with multiple entrances between roughly 8 and 32 metres. Divers progress through wide corridors decorated with red and yellow encrusting sponges, branching yellow Parazoanthus axinellae, and dense colonies of orange Astroides calycularis and red gorgonian Paramuricea clavata on the deeper outer walls. The cave is a known habitat for European lobster (Homarus gammarus), forkbeard, conger eel, slipper lobster, and large groupers (Epinephelus marginatus) that patrol the entrances. Schools of cardinalfish hover in the darker chambers and sea breams cruise the outer reef. Visibility commonly reaches 25 metres in summer and currents are typically weak inside the system. The dive is reserved for experienced divers with cave-overhead awareness, good buoyancy and a torch; multi-level profiles are standard. Capo Caccia and the adjacent Capo Galera headland form one of Sardinia's premier diving areas, protected within the Capo Caccia – Isola Piana Marine Protected Area.