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 Alif Atoll Dhaalu Atoll. The North Ari Atoll administrative division consists of the northern part of the geographic or natural atoll Ari Atoll (described as Northern Ari Atoll in this context to differentiate from the official name of the administrative division), Rasdhukuramathi Atoll and Thoddoo Island. The atolls consists of Inhabited Islands and Uninhabited Island, a definition which includes resort islands, airport islands and industrial islands. Maalhos: Maalhos is geographically the westernmost island of the Atoll and the second-most westerly inhabited island of the Maldives after Maamakunudhoo). Vajrayana Buddhist remains have been found on this island. Rasdhoo: Rasdhoo is the capital island