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Colour satellite image of the Richat structure, Mauritania. This is a geological formation in the Maur Adrar Desert in the African country of Mauritania. Although it resembles an impact crater, the Richat structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock. This image was compiled from data acquired by Sentinel-2 and Landsat 8 satellites. (KEYSTONE/SCIENCE PHOTO LIBRARY/PLANETOBSERVER/SCIENCE PHOTO LIBRARY)