Atoll

A ring-shaped coral reef enclosing a lagoon, formed as a reef keeps growing while its volcanic island sinks; classic examples ring the Maldives.

An atoll is a ring or horseshoe of coral reef enclosing a central lagoon, usually dotted with low sand islands. Darwin worked out the formation: a fringing reef grows around a volcanic island, the island slowly subsides while the coral keeps building toward the light, and eventually only the ring remains around a lagoon where the peak used to be. The Maldives, Tuvalu, and the atolls of French Polynesia and Belize are textbook examples.

For divers, atolls concentrate everything into predictable zones. The outer wall drops steeply into oceanic water and attracts pelagics; the lagoon offers calm, shallow sites; and the channels, or passes, that connect them become rivers of current on each tide. Pass dives at atolls like Rangiroa and Fakarava are famous for gray reef sharks stacked in the incoming flow, and are usually run as drift dives timed to the tide.

Atoll trips are often liveaboard territory, since the best rings lie far from airports. Expect briefings to revolve around tide timing and current strength, and expect the term channel, pass, and corner to define most of the week's sites. Bring a reef hook where local rules allow one, plus solid DSMB skills for the drifting pickups that end each pass dive.