The eye of the Sahara - Richat Structure :

This has got to be one of the strangest places on Earth – but you couldn’t make much of it if you were just walking by. It’s located in a rather remote area, and the few people who noticed something odd about it didn’t know just how odd it really is because it’s hard to figure out from ground level; as a result this 50 km structure didn’t receive much attention until some astronauts made reports about it .

Located in Mauritania, the Eye of the Sahara is not really what you would call a structure, but rather a huge circular formation; it was originally thought to be a crater, but the more recent and accepted theories suggest that it is in fact a product of erosion that took place in geological time.
You can also see it on Google Maps, it’s really a brilliant view, and you can zoom in and out for proportions (coordinates are 21.124217, -11.395569).

 The Richat structure (Sahara, Mauritania) appears as a large dome at least 40 km in diameter within a Late Proterozoic to Ordovician sequence. Erosion has created circular cuestas represented by three nested rings dipping outward from the structure. The center of the structure consists of a limestone-dolomite shelf that encloses a kilometer-scale siliceous breccia and is intruded by basaltic ring dikes, kimberlitic intrusions, and alkaline volcanic rocks

Photo credits : Source1, Source2, Source3