December 1900: a lighthouse on a jagged Atlantic rock has gone dark. When the relief vessel Hesperus finally docks through churning swells, the crew climbs up to find the clocks stopped, the fire dead, meals uneaten, and all three seasoned keepers vanished without a trace. A real-life locked-room mystery on a storm-battered speck of ocean.

This episode strips away the ghost stories to reveal a place far stranger than any myth: the Flannan Isles, or Seven Hunters, 32 kilometers off the Outer Hebrides. We explore the volcanic geology, the drowned Ice Age landscape, the medieval monks who sought holy exile there, and the official 1900 inquiry's verdict that a rogue wave swept the men away. It matters because the truth, grounded in geology, history, and ecology, is more fascinating than the locked-room legend it inspired.

• The islands ...