An atoll is called a "low island" because it is formed of only an exposed ring of coral. The Tuamotu archipelago is the oldest group of Tahitian Islands, geologically speaking. Originally a chain of volcanic islands, the warm waters attracted the coral which formed the reefs around the islands. Eventually the weight of the islands on the moving continental plates caused them to eventually disappear leaving only the coral reef. The coral grows as fast as the atoll sinks.

An atoll generally has 0 - 2 channels (passes), and the currents in these passes can be quite strong. In accordance with the tides and general swells the water enters and exits renewing the limpid lagoons.

It is in these passes that are undoubtedly found the most abundant fauna in Polynesia. Drift dives are generally orgainised to ride the current from outside the reef into the lagoon through these passes.

These dives are not to be missed!