Caroline Atoll is one of over 175 atolls and coral islands in the Pacific Ocean. The English naturalist and geologist Charles Darwin first suggested the theory, still accepted, that atolls originate from calcium carbonate platforms that grow around the cones of extinct, sinking volcanoes. Caroline Atoll's high volcanic island has subsided below sea level; the surrounding coral reef has developed upwards, built by corals, its typically annular shape caused by underwater weathering. The coral reef will continue to develop as long as it is not overtaken by rising sea levels, in which case the corals will perish and the atoll will stop growing.