Why did three lighthouse keepers vanish without a trace in 1900?

From left to right: James Ducat, Thomas Marshall, Donald McArthur and Superintendent Robert Muirhead in 1900 (Picture: Alamy/Steven Gibbons)

There was only thing to greet Captain Jim Harvie when his boat docked at a barren lump of rock in 1900 – a lighthouse.

Harvie blew the boat’s whistle and shot a flare as he came near the isle on that Boxing Day. No response.

His crewmate Joseph Moore stepped out onto Eilean Mòr, an island to the north-west of Scotland, to seek out the three...