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
Northern Thiladhunmathi Atoll there are three Tourist resorts in the atoll. North Thiladhunmath is third largest atoll in the Maldives in terms of population and land area. The atoll being at the north tip of the Maldives is the closest to Sri Lanka and India. Northern Thiladhunmathi Atoll contains 42 islands, 16 of which are inhabited. NOTE: "Haa Alifu, Haa Dhaalu, Shaviyani, Noonu, Raa, Baa, Kaafu, etc. 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 while