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
Haa Alif Atoll Secretariat of North Thiladhunmathi Atoll Council is located. The Northern Thiladhunmathi Atoll administrative division consists of a total of 43 islands spread over two natural geographic atolls, namely Northern Thiladhunmathi, and Ihavandhippolhu. Northern Thiladhunmathi is the northern-most portion of the huge Thiladhunmathi Atoll. Thiladhunmathi Atoll was administratively divided into northern and southern divisions in 1958. Ihavandhippolhu lies to the north of Northern Thiladhunmathi, with the Ihavandhoo Kandu channel separating the two, and is the northern-most atoll of the Maldives. It is a small natural atoll 22 km in length. It has 25 islands lying all around the boundary reef. One