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
Alif Dhaal Atoll found in Ariadhoo Island. Whale sharks are year-round residents of Alif Dhaal Atoll. The South Ari Atoll administrative division consists of the southern part of the geographic or natural Ari Atoll (described as Southern Ari Atoll in this context to differentiate from the official name of the administrative division). The atoll consists of Inhabited Islands and Uninhabited Island, a definition which includes resort islands, airport islands and industrial islands. Resort islands are classified as Uninhabited Islands which have been converted to become resorts. The following are the resort islands, with the official name of the resort. Alif Dhaal Atoll Alifu