The Flannan Isles mystery

The Flannan Isles, to the west of the Isle of Lewis in the Outer Hebrides are not quite as isolated as St Kilda, but are smaller, and non-inhabited. They show up on Google Maps' satellite view, but not the map view. Wikipedia reports:

It is unlikely that there were permanent residents on the islands once the Celtic Church fell into decline in the Hebrides (as a result of 9th century Viking invasions), until the construction of the lighthouse and its occupation very shortly before the dawn of the 20th century. The islands have been devoid of permanent residents since the automation of the lighthouse in 1971. They are the location of an enduring mystery which occurred in December 1900, when all three lighthouse keepers vanished without trace.

Read more about the mystery on Wikipedia.