Tabarka

Tabarka was a French-built steamer sunk as a blockship to seal Burra Sound against U-boats, and the tide that justified her sinking now makes her Scapa's most joyful dive. At slack water divers drop into the inverted hull, where daylight pours through gaps in the plating in cathedral shafts, boilers and engine stand like furniture in a green-lit hall, and every surface is upholstered in anemones fed fat by the current. Seals sometimes flash through the openings. The window is short and the exit non-negotiable, but few UK dives pack this much light and life into 18 m.

The moment you come for: Light shafts inside an upturned hull, anemone-carpeted steel, seal cameos at slack

Topography: An upturned steamer hull in clean tidal shallows, interior open and washed by light through hull openings

Current: Outside the brief slack window, current in the sound reaches speeds no diver can swim against; timing is everything.

Entry: Dived only at slack water in Burra Sound, where the tide otherwise runs far too hard to hold position.

Hazards: Ferocious current outside slack; overhead sections require care and an eye on the exit windows.

Conservation: The blockships are historic remains in a strong-tide environment; take nothing and leave the resident life undisturbed.