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
Gnaviyani Atoll a whole. NOTE: "Haa Alifu, Haa Dhaalu, Shaviyani, Noonu, Raa, Baa, Kaafu, etc.. (including Gnaviyani) are code letters assigned to the present administrative divisions of the Maldives. They are not the proper names of the natural atolls that make up these divisions. Some atolls are divided into two administrative divisions or contain half of an atoll (In case of Thiladhunmathi to Haa Alif, Haa Dhaal, Shaviyani and Noonu Atolls, Huvadhu Atoll to Gaafu Alifu and Gaafu Dhaalu Atolls and In case of half of Ari Atoll to Alifu Dhaalu Atoll), while other divisions are made up of two or more