Rangiroa is a ring of 415 sand islets around a lagoon big enough to hold the whole island of Tahiti. It lies in the Tuamotus of French Polynesia, and the ring is what a volcano left behind: as the mountain sank, the coral on its flanks kept climbing to stay in the light. The lagoon is the space the mountain used to fill. Only two gaps break the ring, and twice a day the Pacific pours through them to empty and refill everything inside. That is where the life of the atoll collects. On the incoming tide you drop into the ocean outside Tiputa Pass and let the current carry you in, hanging weightless while grey reef sharks stack up by the dozen in the flow and eagle rays cut across it. The ride lasts minutes, and the boat picks you up on the lagoon side.
Inside the ring the same water is unrecognisable. Boat trips cross an hour or more of it to the far shores: the Blue Lagoon, a shallow pool of electric blue ringed by tiny motu, and the pink-tinted sand beyond. You wade and snorkel there over coral heads in water no deeper than your chest. Other motu are left to the birds; you step onto the white sand of one and have it to yourself and several hundred terns, noddies and boobies. The inhabited land is a thread. A bicycle covers the strip between the two passes, ocean surf breaking on one side and flat lagoon on the other, past pearl farms, small churches and family gardens. The farms raise Tahiti's black pearls inside the black-lipped oyster, which thrives in this warm, plankton-rich water. One motu grows wine on soil spread over bare coral; with no winter to make them rest, the vines fruit two or three times a year. Hammerheads hunt the pass from December to March, mantas from July to September. The dolphins keep no season: a resident pod lives at the gap and rides the current on every tide, swinging in to look the divers over.