FitzRoy's instructions from the Admiralty required a detailed geological survey of a circular coral atoll to investigate how coral reefs formed, particularly whether they rose from the bottom of the sea or the summits of extinct volcanoes, and the effects of tides measured with specially constructed gauges. He chose the Keeling Islands in the Indian Ocean, and on arrival on 1 April, the entire crew set to work. Darwin found a coconut economy, serving both the small settlement and wildlife. There was a limited range of native plants and no land birds, but hermit crabs everywhere. The lagoons teemed with a wide variety of invertebrates and fish, and he examined the atoll's structure in view of the theory he had developed in Lima, of encircling reefs becoming atolls as an island sank. This idea was supported by the numerous soundings FitzRoy had taken, showing a steep slope outside the reef with no living corals below 20–30 fathoms (40–60 m).