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
Meemu Atoll of the natural atolls that make up these divisions. Some atolls are divided into two administrative divisions while other divisions are made up of two or more natural atolls. The order meemu followed by the code letters is from North to South, beginning with the first letters of the Thaana alphabet used in Dhivehi. These code letters are not accurate from the geographical and cultural point of view. However, they have become popular among tourists and foreigners in the Maldives who find them easier to pronounce than the true atoll names in Dhivehi, (save a few exceptions, like Ari Atoll)."