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
Baa Atoll Biosphere Reserve the environment. Baa Atoll has a warm and humid tropical climate. The weather is dominated by two monsoon periods – a rainy monsoon period, which lasts from May to November, and a dry monsoon period, which lasts from January to March. The dominant natural environment of the reserve is marine, surrounded by deep ocean. The water depth varies considerably and includes lagoons with depths ranging from 30 m to 80 m, opening into the Indian Ocean, while channels running through the atoll may be up to 250-300 m in depth. The lagoons enclose a variety of reef structures including faros,