Darwin's Sinking Mountain
In April 1836, a young Charles Darwin arrived at the Cocos (Keeling) Islands aboard HMS Beagle. His visit to this remote coral atoll, though brief, provided direct evidence for a radical new idea. At the time, the formation of these ring-shaped islands was a geological puzzle. Darwin theorized that atolls were the final stage in a long process that begins with a volcanic island. He proposed that as the volcanic island slowly sinks, the coral reefs fringing its shores continue to grow upwards, keeping pace with the subsidence. The reef transitions from a fringing reef to a barrier reef and, once the volcano submerges completely, only the ring of coral remains as an atoll.
To test this, Darwin and the Beagle's crew took depth soundings off the atoll's coast. Just 2,000 meters from shore, their 2,194-meter sounding line could not find the bottom. This confirmed the presence of a steep underwater mountain, just as his theory predicted. He collected coral specimens and observed how the undercutting of coconut trees at the shoreline suggested the land was sinking. He published his findings in "The Structure and Distribution of Coral Reefs" in 1842, years before his more famous work on evolution. While modern science recognizes that fluctuating sea levels also play a part, Darwin's fundamental model of a subsiding volcanic base, confirmed by deep drilling on Pacific atolls in the 1950s, is a foundation of geology.