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At roughly 8 km (5 mi) wide, Atafu Atoll is the smallest of three atolls comprising Tokelau. Land area is only about 2.5 sq km and the lagoon's area is about 15 sq km. The primary settlement on Atafu is a village located at the northwestern corner of the atoll – indicated by an area of light gray dots in the left part of this photograph. The typical ring shape of the atoll is the result of coral reefs building up around a former volcanic island. Over geologic time, the central volcano subsided beneath the water surface, leaving the fringing reefs and a central lagoon that contains submerged coral reefs. Image courtesy of NASA.