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This, like natural selection, was pure speculation on Darwin's part. Atoll-formation was a phenomenon of history, and not something that he could "prove" with 19th century technologies. In fact, it was not until the 1951, when the U.S. Geologic Survey drilled Enewetak Atoll in the Marshalls to 1,340 meters, that Darwin's theory was confirmed: deep down, below the layer of coral in the atoll lagoons, the core samples revealed the volcanic rock of a former island.

In the clear waters of Indonesian reefs, ultraviolet radiation can penetrate several meters underwater. Pigments made up of amino acids, called S-320, shield the delicate growing tips of shallow-water corals such as this Acropora sp.

Step 1. A fringing reef forms around an island.

Step 2. The island sinks, and the fringing reef grows into a barrier reef.

Step 3. The island sinks below the surface, and only an atoll remains. (After Darwin, 1842)

Step 1. Limestone is exposed by geological forces.

Step 2. Rainfall erodes exposed limestone.