Charles Darwin is best known as an evolutionary biologist but he also had significant success as a geologist.  His first three scientific books after his account of the voyage of the Beagle explored the geology of coral reefs, volcanic islands and South America: The Structure and Distribution of Coral Reefs (1842); Geological Observations of Volcanic Islands (1844); and Geological Observations on South  America (1846).

Dr Brian Rosen, a Scientific Associate in the NHM Department of Zoology, gave an invited public lecture, hosted by the Bermuda Institute of Ocean Sciences (BIOS) in their ‘Distinguished Lecture Series’ in November 2010, entitled ‘Red or Blue? Darwin's Bermuda Dilemma and his Enduring Subsidence Theory of Coral Reefs’, in which he discussed Darwin's ideas and conclusions.

Darwin's subsidence theory of the origin of coral atolls was based on the idea that the world's ocean floors as a whole were subsiding, and that atolls had developed as coral growth kept pace with sea level on the sinking foundations of former volcanoes. The sheer scale and simplicity of Darwin's idea was reflected in his famous coral reef map, on which he shaded atolls and most other oceanic reefs in blue (inferring subsidence), and most reefs elsewhere in red (inferring uplift or stability).

Darwin's theory was initially acclaimed but over time it seemed too revolutionary for many - alternative ideas such as sea level changes were preferred.  However, by the mid twentieth century, it became clear that the reef deposits of most atolls did indeed extend hundreds of metres beneath their surfaces - far deeper than can be explained by glacial sea level changes alone. 

We know now that atolls do form because islands sink gradually as a  consequence of tectonic plate movement.  While Darwin was correct about  the subsidence, tectonic plate movement and sinking as a reason was not proposed until 1912, althougth this was hotly contested until scientific explanation in the 1950s and 1960s.

Although Darwin did not visit Bermuda, he included it in his coral reef book. Bermuda's reefs - being essentially ring-like in arrangement, and with a central lagoon - are atoll-like, but for other reasons Darwin was equivocal about whether it was a true atoll, so he left it uncoloured on his reef map. In his lecture Brian Rosen attempted to resolve Darwin's dilemma, based on the fact that the sea levels at Bermuda have fluctuated over time.