Flat Holm was an island located south-southeast of Cardiff Bay in the middle of the Bristol Channel.

Assumed to be uninhabited scrubland, the island was secretly used by Jack Harkness to care for and keep safe those taken in by the Rift and returned, scarred mentally (and sometimes physically). When he took over Torchwood Three in 2000, Jack found two survivors in the vaults. He created a place where they could be cared for, calling them experiments which had gone wrong to the carers. Jonah Bevan ended up there, as Gwen Cooper discovered. Jack had kept the facility secret from her. (TV: Adrift)