Fingal’s Cave, Inner Hebrides, Scotland. "Fingal's Cave is a sea cave on the uninhabited island of Staffa, in the Inner Hebrides of Scotland, part of a National Nature Reserve owned by the National Trust for Scotland" via Wikipedia.
open cave, ahrax mellieha, Mellieha, Malta - Recommended by a friend and situated in the North of the island. Reachable by canoe/boat from Armier bay from the left. Another hidden gem in the little island of Malta!