Catastrophic geological events that suddenly change the cartographic face of Earth are as rare as they are spectacular. When the isthmus that joined Iberia to North Africa was breached some 6 million years ago, the resulting superflood created the Strait of Gibraltar and refilled the long-dessicated Mediterranean basin in just a few decades. Some geologists argue that the Bosporus strait at Istanbul is the result of a similar breach eight thousand years ago: Mediterranean waters spilling into the lower-lying, previously landlocked Black Sea, they speculate, suddenly raised its level, sweeping away human communities around its littoral and giving rise to the flood myths of Gilgamesh and Noah.

In 1985 Alec Smith at Royal Holloway College London invoked a fragmentary seismic survey of the English Channel’s floor to conjecture that the Dover Strait, which links the Channel to the North Sea, was created a few hundred thousand years ago by a...

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