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
Southern Maalhosmadulu Atoll resorts. Thulhaadhoo Island is traditionally well known for its lacquerwork handicrafts. Maalhosmadulhu Atoll is also considered as a good example of the rich biodiversity found in the Maldives, including large mangroves and a unique diversity of fauna, such as the benthic fauna. Furthermore, the ring-shaped reef forms known as "faru" in the local language is a reef structure which is unique to the Maldives. The southernmost uninhabited island of Olhugiri in the Baa atoll lies 13 km north of Goifulhafehendhu Atoll. Olhugiri is well known for its unique natural vegetation and for providing two of the only perching sites for