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
Addu Atoll Addu Atoll Unlike other atolls of Maldives, Addu city possesses a natural anchorage within the city basin, as the atoll is land-locked with large islands surrounding the atoll. This results in a natural harbor that is very calm and safe for sea vessels at all times, and is not affected by seasonal changes. The atoll possesses four channels which lead into the lagoon. In the north are the Kuda Kandu and Maa Kandu, in the south the Gan Kandu, and the broad Villingili Kandu is to the southeast. There are unique features in the islands of Addu Atoll. The islands