Atoll remains of an ancient atoll as a hill in a limestone area is called a reef knoll. The second largest atoll by dry land area is Aldabra with 155 km². The largest atoll in terms of island numbers is Huvadhu Atoll in the south of the Maldives with 255 islands. In 1842, Charles Darwin explained the creation of coral atolls in the southern Pacific Ocean based upon observations made during a five-year voyage aboard from 1831 to 1836. Accepted as basically correct, his explanation involved considering that several tropical island types—from high volcanic island, through barrier reef island, to atoll—represented
Dongsha Atoll National Park a circular atoll in diameter, with tropical monsoon climate. It is administered under Qijin District, Kaohsiung City. Seagrass Beds are well formed in the atoll and in adjacent waters, providing rich bio-diversities of marine life from fish, jelly fish, squid, sicklefin lemon sharks, and rays to rarer sea turtles, Dugongs, and cetaceans (dolphins and whales). Recovery of green sea turtles has especially been noted. Possible use of the atoll as a breeding ground by lemon sharks have been speculated due to the numbers of infants that have been discovered. Dongsha Atoll National Park The Dongsha Atoll National Park () is