Neptune's Grotto is the dramatic sea cave at the foot of the Capo Caccia headland, twenty-four kilometres north-west of Alghero. The cave system extends four kilometres into the limestone cliff and contains a freshwater lake — Lake Lamarmora — surrounded by stalactites and stalagmites, some over a thousand years in formation. The acoustics are extraordinary, occasionally used for chamber concerts.
There are two ways in. The boat trip from Alghero port takes about an hour each way and is the more scenic option, especially in late afternoon when the cliffs glow gold. The land approach is the descent of the Escala del Cabirol, 654 steps cut into the cliff face — spectacular going down, brutal coming back up. Visits are guided only; tours run hourly in summer, less often off-season, and don't run at all if the sea is too rough.