Charles Darwin, Marine Geologist

Atolls are circular coral reefs that enclose a lagoon. Many atolls are found in deep ocean waters, far from continental shorelines, and their formation had long been a mystery.

Charles Darwin encountered a number of atolls during the 5-year Beagle voyage, and after observing the variety of coral reef forms, from fringing reefs ringing volcanic islands, to the circular atolls seemingly rising from the sea floor without benefit of a companion island, he puzzled out their formation.

Darwin correctly interpreted these different reef forms as having a common origin, both starting out as a reef in the shallow waters around a newly formed volcanic island. When volcanism ended, the island volcano began to sink. The coral had to grow upward to remain in shallow water for its survival. As the volcano subsided beneath the waves, the ring of coral that once marked its perimeter remained as an atoll.

Photo credit: http://www.coral-reefs.org/Apron-Fringing-Barrier-Patch-reef.html