Which atoll did Darwin study to support his theory of coral reef formation?

Charles Darwin studied the Cocos (Keeling) Islands to support his theory of coral reef formation.

During the voyage of HMS Beagle, Darwin examined the Keeling Islands in the Indian Ocean in April 1836. The group is a coral atoll: a ring-shaped reef and islands surrounding a lagoon. Darwin used its structure, together with observations from other Pacific islands, to develop his subsidence theory.

He proposed that a volcanic island could slowly sink while coral continued growing upward in shallow water. A fringing reef would become a barrier reef, and eventually an atoll could remain after the central island disappeared beneath the sea. The deep lagoon and limited depth at which reef-building corals could live helped support this explanation.

A common mix-up is calling the site simply “Cocos Island.” The official name is the Cocos (Keeling) Islands, and the relevant feature is the Keeling atoll. Darwin published his conclusions in The Structure and Distribution of Coral Reefs in 1842.