This cave was born in the mountain and found itself underwater. Its galleries have the shape typical of cavities cut by fresh water under pressure, not by the sea: which means it formed while it was dry, inside an aquifer, and that the Mediterranean came later to fill it. Among sea caves it is a rarity.
What to see
It opens on the eastern side of the limestone massif of punta Giglio, a few kilometres north-west of Alghero, in the same karst system that holds other famous submerged cavities. The limestone here is bright white and pierced in several places, and it is exactly those holes that suggested the name.
The diver who found it in 1994 said the rocks reminded him of the ghosts in comics, the ones made with a sheet and two eyeholes. It is not a solemn christening, and for that reason it says a great deal about how the underwater world was discovered: mapped only recently, and by people still free to choose the names.
Visiting
You can only get there by diving, with a diving centre and the right certification, following the rules of the protected area; from land there is nothing to see. Half a day with the boat trip.
Our editors' tip
Underwater, point your torch sideways, not straight ahead. Frontal light flattens the wall and the holes disappear; raking light brings them all out at once, and only then do you understand why they looked like sheets to somebody. It holds for this cave and for any other.