Otemanu, the 727-metre blade of dark basalt at the centre of Bora Bora in French Polynesia, is slowly sinking, while the coral reef ringing the island keeps building upward. The lagoon is the gap between the two, and it is widening. One distant day the mountain will be gone and only the ring will be left. This is an island caught halfway through its life. Staying here usually means staying over the water. Wooden bungalows on stilts put the lagoon directly under the floor, warm enough to swim at any hour. Most have a glass panel set into the boards with a light beneath it, and the fish come to it after dark. Bora Bora did not invent any of this. Three American hoteliers built the first overwater bungalows on neighbouring Raiatea in 1967, on a reef with no beach of its own, and this lagoon is what turned the idea into the picture of the South Pacific.
An outrigger runs out to a shallow sandbar where southern stingrays glide up against your legs and blacktip reef sharks circle in close, the water warm enough to stand waist-deep among them. Carrying on round the island shows you what the road cannot: from the water Otemanu and Pahia are sheer faces, not the rounded humps they look from land. The boat finishes at a motu on the reef ring, a strip of sand with nobody on it, ocean surf breaking on one side and flat lagoon water on the other. Inland is a different island again. A rough back road climbs past small shrines to American coastal guns rusting on the ridges, left from 1942, when this was the first United States forward base of the Pacific war. Not a shot was fired here in anger. The one beach with no gate is Matira, on the southern tip. It shelves so gently that at dusk you can wade a long way out and still be waist-deep, with Otemanu laying its shadow across the water behind you.