Precontinent II (Conshelf Two)

In the summer of 1963, Jacques Cousteau's team lived for a month on this reef in an underwater village - Precontinent II, the Conshelf Two experiment - and when they left, they left pieces of it behind. The star relic is the submarine hangar, an onion-domed steel garage at nine meters that once housed the diving saucer; swim up inside and your exhaled breath pools against the dome as a wobbling silver mirror, exactly as the aquanauts' did. A tool shed, fish pens, and scattered frames sit nearby, all upholstered in decades of coral. As a dive it is shallow, sheltered, and easy; as a place it is unmatched - the only spot on earth where recreational divers casually tour the ruins of humanity's first serious attempt to live in the sea.

The moment you come for: The onion-dome hangar and its trapped-air mirror; Cousteau-era relics under coral; effortless conditions inside the lee.

Topography: Reef slope holding the steel structures of a 1963 underwater habitat experiment

Entry: On Sha'ab Rumi's sheltered western side, inside the reef's lee; often dived as an easy afternoon or check dive.

Hazards: Trapped air under the dome is not breathable gas; mild surge near the reef crest.

Conservation: The structures are irreplaceable heritage - no scraping names, no souvenir chips of dome paint.