Darwin’s theory of atoll formation was that as the volcanic portion of the island subsides, the fringing reef is converted into a barrier reef. After the volcanic core has disappeared completely into the lagoon, the remaining reef island is called an atoll
Raroia, the atoll where the Kon Tiki ended its journey while trying to prove the theory that Polynesians descended from South America has only one narrow entrance with turbulent currents and overfalls. Navigation once inside is fraught with danger due to numerous coral heads through unchartered waters that raises vertically to near the surface waiting to claim your boat. Diving is the primary reason you come to this part of the world, and the remoteness of this inlands force you to partly hibernate or the least to train your brain to stay in neutral for long period of time.
Fakarava is a larger atoll with two navigable entrances to the lagoon. Black pearls are farmed in most of the Tuamotus and are unique to these islands.