The Richat Structure (Eye of the Sahara), Mauritania — a giant bullseye pattern in the desert, about 25 miles across, so perfectly circular that early astronauts used it as a landmark to spot from orbit.
https://www.reddit.com/r/Damnthatsinteresting/comments/1w9vf3e/the_richat_structure_eye_of_the_sahara_mauritania/