Coral reef A fringing reef forms around an extinct volcanic island as the island and ocean floor subsides. As the subsidence continues, the fringing reef becomes a barrier reef and ultimately an atoll reef. Darwin predicted that underneath each lagoon would be a bedrock base, the remains of the original volcano. Subsequent research supported this hypothesis. Darwin's theory followed from his understanding that coral polyps thrive in the tropics where the water is agitated, but can only live within a limited depth range, starting just below low tide. Where the level of the underlying earth allows, the corals grow around the coast
The Structure and Distribution of Coral Reefs of the records of sea temperature kept in the ship's "Weather Journal". He had his first glimpse of coral atolls as they passed Honden Island on 9 November and sailed on through the Low or Dangerous Archipelago (Tuamotus). Arriving at Tahiti on 15 November, Darwin saw it "encircled by a Coral reef separated from the shore by channels & basins of still water". He climbed the hills of Tahiti, and was strongly impressed by the sight across to the island of Eimeo, where "The mountains abruptly rise out of a glassy lake, which is separated on all sides, by a