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What resembles an impact crater on an exotic moon is actually a geographic oddity on our planet. Some 30 miles (50 km) wide, the Richat Structure in the Sahara Desert in Mauritania is easily seen by passing space shuttles and satellites. This dramatic shot came from the Landsat 7 orbiter. Geologists remain uncertain what formed the ring-shaped feature, but have dismissed a meteor collision or volcanic outburst as the culprit.