It is the largest submerged sea cave in the Mediterranean, and nobody knows for certain who found it. The reason is its own size: it has many openings and two main entrances very far apart, so in 1957 it was discovered several times over, by different divers going in at different points without knowing they were in the same place.
What to see
It opens beneath the limestone massif of capo Caccia, inside the marine protected area, a few hundred metres from the grotta di Nettuno. It is a system of galleries and chambers with high vaults, colonies of red coral, sponges, and the light that comes in at the mouths and fades slowly as you go deeper.
The first to explore it were coral fishers: they were after red coral and meanwhile recovering from the bottom the shells left there by the second world war. One of them tore from the vault a block of coral weighing two and a half quintals, which today would be a crime and was then a trophy. The name was chosen for the father of the Nereids, to pair with the Neptune of the cave next door.
Visiting
It is a dive, not a visit: you go in only with a diving centre, with the right certification and following the rules of the protected area. From land you see nothing. Half a day counting the boat trip.
Our editors' tip
If you do not dive, go to the grotta di Nettuno, which is close by and can be visited on foot down the stairway on the cliff or by boat from Alghero. It is the same rock, the same karst system: what you see dry above goes on underwater for hundreds of metres.