Flannan Isles Lighthouse is near the highest point on Eilean Mòr, one of the Flannan Isles in the Outer Hebrides off the west coast of Scotland. It is best known for the mysterious disappearance of its keepers in 1900. The keepers were: James Ducat, Donald McArthur and Thomas Marshall.
Subsequent researchers have taken into account the geography of the islands. The coastline of Eilean Mòr is deeply indented with narrow gullies called geos. The west landing, which is situated in such a geo, terminates in a cave. In high seas or storms, water would rush into the cave and then explode out again with considerable force. It was possible MacArthur may have seen a series of large waves approaching the island, and knowing the likely danger to his colleagues, ran down to warn them only to be washed away as well in the violent swell.Recent research by James Love discovered that Marshall was previously fined five shillings when his equipment was washed away during a huge gale. It is likely, in seeking to avoid another fine, that he and Ducat tried to secure their equipment during a storm and were swept away as a result. The fate of MacArthur, although required to stay behind to man the lighthouse, can be guessed to be the same. Love speculates that MacArthur probably tried to warn or help his colleagues and was swept away too. This theory also has the advantages of explaining the set of oilskins remaining indoors and MacArthur's coat remaining on its peg, although perhaps not the closed door and gate. Another theory is based on the first-hand experiences of Walter Aldebert, a keeper on the Flannans from 1953 to 1957. He believed one man may have been washed into the sea but then his companions, who were trying to rescue him, were washed away by more freak waves.
A further proposal is based on the psychology of the keepers. Allegedly MacArthur was a volatile character; this may have led to a fight breaking out near the cliff edge by the West Landing that caused all three men to fall to their deaths. Another theory is that one of the men went insane (perhaps MacArthur, as evidenced by him leaving the lighthouse without his rain gear and his strange behaviour documented in the log book), murdered the other two, threw their bodies into the sea, and then jumped in to his own death.