Grotta di Nereo

The dive

Grotta di Nereo runs into the limestone of Capo Caccia at the north-west corner of Sardinia, inside the Capo Caccia - Isola Piana marine protected area. Published descriptions agree on the scale: a main gallery of roughly 300 m, with secondary tunnels, side branches and chambers taking the total development to about half a kilometre, and a maximum depth of 33 m. It has been dived since 1957, when the first divers in also stripped a substantial red coral deposit out of it. Portoconte's dive centre page counts ten openings and puts the range at 0 to 33 m; the marine park's own itinerary names three main entrances, at 20 m, 18 m and 32 m, the 18 m one being the opening divers call the Archi di Nereo.

The usual guided route is a circuit rather than an out-and-back. Capo Galera describe going in at the deepest entrance at the foot of the cliff, past red coral, up through a tunnel into a lit chamber, then along a passage more than ten metres wide with a white sand floor, and out through a large window at 18 m, the loop closing at about 23 m. Portoconte describe the layout as an A: two main chambers linked by a short two-level passage, the lower level at about 25 m with rocky floors and pebbles of white limestone and red coral, and by a longer tunnel of roughly 200 m that runs from 26 m in the north hall up to around 10 m, with two internal siphons that surface inside the cave. The vaults of the north hall are covered in yellow Parazoanthus and lime-yellow Leptopsammia with patches of red coral.

This is a real overhead environment. Only the ground near the entrances is lit; the inner tunnels are completely dark and the park's own description says the dive has to be organised as a cave dive with cave equipment. The regulation is written accordingly. Nereo sits in zone B of the marine protected area, diving needs prior authorisation from the managing body, and the cave may only be entered by a diver holding at least a first-level cave specialty certification (Cavern Diver or equivalent) on top of a second-level recreational certification. Guided visits are run by authorised diving centres, capped at ten divers per dive and three dives a day at each authorised Nereo point, daylight only.

One thing worth knowing before you book: the marine park does not treat Nereo as a single dive. Its 2025 rulebook lists four separate authorised cave points at Capo Caccia -- Nereo/fattucchiera, Nereo/tunnel, Nereo/cattedrali/fiordo and Nereo/porticato -- each with its own coordinates and its own daily quota. Which of them you are taken to determines what you actually see.

• Vaults of the north hall carpeted in yellow Parazoanthus and Leptopsammia
• Red coral in the transition zones, shallow by Mediterranean standards
• The 18 m opening known as the Archi di Nereo
• A tunnel over ten metres wide with a white sand floor
• Spiny lobsters, European lobsters, conger and moray in the dark passages

Conditions

A boat dive under the Capo Caccia cliff, run as a guided cave dive. Daylight reaches only the ground immediately inside the entrances; past that the passages are completely dark and lights are the whole visual experience. Floors alternate between rock, white limestone pebbles and white sand, and two internal siphons in the long tunnel rise to the surface inside the cave.

On current: Sources put the deepest entrance at 32 m (marine park itinerary, Capo Galera) or 33 m (Portoconte, Wikipedia), and one summary gives 35 m; plan on the deeper figure. None of the sources read describe current as a factor inside the cave itself, and none give a visibility figure, so neither is recorded here.