Il tunnel

The dive

This pin sits within about 20 m of the position the Capo Caccia - Isola Piana marine protected area publishes for the authorised cave point it calls Nereo/tunnel, at 40 33.749N 8 9.700E. The park's 2025 rulebook lists four separate authorised entries into the Grotta di Nereo -- fattucchiera, tunnel, cattedrali/fiordo and porticato -- each with its own coordinates and its own quota of ten divers per dive and three dives a day. So this is not a separate cave. It is one permitted route into the largest submerged sea cave in the Mediterranean, and everything the park requires for Nereo applies here unchanged.

The tunnel itself is the feature Portoconte's dive centre calls the via lunga: a long, wide passage of roughly 200 m connecting the cave's two main chambers, starting from about 26 m in the large northern hall and rising to an average of around 10 m, with two broad internal siphons that reach the surface inside the rock. Capo Galera describe the same kind of passage from the diver's side -- more than ten metres across, floored in white sand, exited through a large window at 18 m with the loop closing near 23 m. Portoconte put three entrances on the northern side of the cave: the biggest at 33 m, about 15 m wide and 5 m high and facing north, plus two more at roughly 11 to 15 m. Since Nereo/tunnel is the northernmost of the park's four Nereo coordinates, that northern entrance complex is the most likely way in, though no source states the pairing outright.

Everything past the entrance is dark and overhead. The park's own itinerary for Nereo says only the ground near the openings gets natural light and that the dive has to be run with cave organisation and cave equipment. Legally, diving here needs prior authorisation from the managing body, a second-level recreational certification, and at least a first-level cave specialty (Cavern Diver or equivalent); guided visits are run by authorised diving centres in daylight only.

One name trap worth knowing. There is a different Alghero dive also called Il Tunnel: Capo Galera's 80 m passage at a maximum of 18 m that cuts right through the promontory of the sommergibile bay, which the park lists separately as Tunnel Azzurro at Cala Puntetta, 40 36.149N 8 9.693E, about 4.4 km north of this pin. It is a shallow, non-cave-certified swim-through and has nothing to do with Nereo. Check which one a centre means before you book.

• A connecting tunnel of roughly 200 m running from about 26 m up to around 10 m
• Two wide internal siphons that surface inside the rock
• A passage over ten metres across with a white sand floor
• The 33 m northern entrance, about 15 m wide and 5 m high

Conditions

A guided cave dive off a boat: down the cliff to the entrance, through dark, wide passage on lights, and up through the tunnel toward the shallower openings. The floor alternates between rock and white sand. Nothing in the sources read suggests current or surge inside the cave is a routine factor.

On current: No source read gives a current or visibility figure for this point. Depth for the northern Nereo entrance is given as 33 m by Portoconte and 32 m by the park itinerary and Capo Galera; plan on the deeper figure.