Richat Structure, Mauritania
This prominent circular feature in the Sahara desert of Mauritania 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. Described by some as looking like an outsized fossil in the desert, the structure has a diameter of almost 30 miles.
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https://www.nasa.gov/image-detail/richat-structure-mauritania-2/
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