The Richat Structure, near Ouadane, Mauritania

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The Richat Structure is a prominent circular feature in the Sahara desert of Mauritania near Ouadane. It has attracted attention since the earliest space missions because it forms a conspicuous bull’s-eye in the otherwise rather featureless expanse of the desert. The structure, which has a diameter of almost 50 kilometres (30 miles), has become a landmark for space shuttle crews.