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
Baa Atoll Baa Atoll Baa Atoll (includes Southern Maalhosmadulu Atoll or Maalhosmadulu Dhekunuburi, Fasdhūtherē Atoll, and Goifulhafehendhu Atoll) is an administrative division of the Maldives. It consists of three separate natural atolls, namely southern Maalhosmadulu Atoll (which is 42 km long and 32 km wide and consists of 9 inhabited islands), the Fasdūtherē Atoll (wedged in between the two Maalhosmadulu Atolls and separated from north Maalhosmasdulu Atoll by Hani Kandu or Moresby Channel) and the smaller natural atoll known as Goifulhafehendhu Atoll (Horsburgh Atoll in the Admiralty charts). Situated on the west of the Maldives atoll chain, it consists of 75 islands