Grotta di Nettuno in Alghero

You're standing at Neptune's Grotto, on the Capo Caccia headland just west of Alghero. Look up along the rock face. That cliff rises about a hundred and ten metres, and right at the bottom, barely a metre above the water, is the entrance to the cave. That's why you can only go in when the sea is calm. When the waves are up, the grotto stays closed.