The Richat Structure in the Sahara Desert of Mauritania is clearly visible from space because it is nearly 50 kilometers across. Previously thought to be an impact crater, the flat middle and lack of shock-altered rock indicates otherwise.
Images © NASA
Why the Richat Structure is nearly circular remains a mystery.
The possibility that it was formed by a volcanic eruption also seems improbable because of the lack of a dome of igneous or volcanic rock. Rather, the layered sedimentary rock of the Richat structure is now thought by many to have been caused by uplifted rock sculpted by erosion. The above image was captured by the ASTER instruments on board the orbiting orbiting Terra satellite.