Stare into the eye of Africa's unfathomably huge Richat StructureS

At close to 50 kilometers in diameter, the Sahara Desert's sprawling Richat Structure is one of the most mysterious geological features on Earth.

Via NASA:

Once thought to be an impact crater, the Richat Structure's flat middle and lack of shock-altered rock indicates otherwise. The possibility that the Richat Structure 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.

Why the structure is nearly circular, however, remains a mystery.

Top photograph by Dutch astronaut Andre Kuipers via ESA/NASA