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
Oroluk Atoll Oroluk Atoll Oroluk Atoll is an atoll belonging to Pohnpei State in the Micronesia. Oroluk Atoll stretches from the northwest to the southeast with a length of about and an average width of . The lagoon's surface is roughly . The more than 25 sandy islets and banks, predominantly on the eastern rim of the atoll, have been washed away by cyclones through the years. Only one island remains, Oroluk Island in the very northwest corner of the atoll. Oroluk Atoll is believed to have been first discovered and named by navigators from Namoluk Island in the Mortlock Islands. The