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
Tema Reef Tema Reef Tema Reef, also called Tima Reef, is a submerged coral reef in the northern Cook Islands, southeast of Pukapuka. No coral or rock shows above water, but the sea breaks heavily over the reef. The reef measures about , covering a shallow but submerged area of . Legend has it that Tima, an Aitutakian warrior sailed to Nassau, but was chased away by Ngalewu, a chief from Pukapuka. On his return to Aitutaki he passed the reef nearby and named it "Te Toka-O-Tima", the Stone of Tima The reef was sighted by Commodore John Byron on June 21,