Forty kilometres of concentric rings in the Mauritanian desert, so regular that from the ground you would never notice and from orbit you cannot look away; astronauts have used it as a landmark since the earliest crewed flights. For decades it was assumed to be an impact crater, and you will still hear that, along with a persistent claim that it is Atlantis. The geology does not support either. Investigators found no shock metamorphism, and the coesite once reported turned out to be misidentified barite. What the work by Matton and colleagues in 2005 and 2008 describes instead is a collapsed volcanic dome: an alkaline igneous body pushed up from below in the Cretaceous, 98 to 104 million years ago, ring faults and gabbro dikes forming around it, and a hundred million years of erosion slicing the layers flat to leave the rings exposed. Théodore Monod documented it properly in 1952. Near Ouadane, Adrar Region.
Adrar Region, Mauritania
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