Also known as the Richat Structure, the Eye of the Sahara located in Sahara, Mauritania was originally thought to be a crater. This large dome is at least 40 km in diameter. Geologists attribute it’s formation within a Late Proterozoic to Ordovician sequence. They believe erosion over time created circular cuestas design three nested rings dipping outward from the structure.
Still other argue that the dome is too circular not to have been human-made, and still other suggest it resembles tales of the mythic Atlantis. You can see the Eye of the Sahara on Google Maps, very clear and brilliant view, and you can zoom in and out for proportions (coordinates are 21.124217, -11.395569).