my great-uncle was a lighthouse relief keeper and the Flannan Isles thing scared him for life

This one's family, second-hand, so take it as that. The Flannan Isles mystery — Eilean Mòr, off the Outer Hebrides, Scotland, December 1900, three lighthouse keepers (Ducat, Marshall, MacArthur) just GONE. No bodies. Logbook had weird entries about a terrible storm when the seas around had been calm……