Which atoll group did HMS Beagle survey to support Darwin's reef theory?

HMS Beagle surveyed the Keeling Islands to support Darwin’s theory of reef formation.

The Keeling Islands, also called the Cocos (Keeling) Islands, are an atoll group in the Indian Ocean. Captain Robert FitzRoy selected them because the Admiralty wanted detailed observations of a circular coral atoll, including its lagoon, outer reef, tides, and depth of living coral.

The Beagle arrived on 1 April 1836. FitzRoy organized boat surveys and soundings around the atoll, while Darwin examined coral fragments and the forms of the seabed. Their observations showed abundant coral growth near the wave-battered outer reef and little or no living coral at greater depths.

Darwin used these findings in The Structure and Distribution of Coral Reefs, published in 1842. His subsidence theory proposed that a volcanic island could gradually sink while its surrounding coral reef continued growing upward, producing a barrier reef and eventually an atoll. The Tuamotus were important elsewhere in his voyage, but the detailed atoll survey tied to FitzRoy’s instructions was at the Keeling Islands.