Blue Eye in the sahara

              In the midst of the vast, vacant Sahara desert, just outside of Ouadane, Mauritania, lies a 30-mile wide geological oddity known the Richat Structure, sometimes called the “Eye of Africa.” From space, this natural curiosity forms a distinct and unmistakable bull’s-eye that once served as a geographical landmark for early astronauts as they passed over the Sahara.

Over time, desert weather has caused the dome to gradually shed layers, resulting in the structure’s remarkable flatness.Its circular shape originally led experts to believe it formed from a meter impact, but NASA says "it is now thought to be merely a symmetrical uplift that has been laid bare by erosion." 

Kediet ej Jill Mountain, Mauritania's highest peak (3,280 feet tall), is nearby, and the mountain has a large concentration of magnetite, giving it a blueish tinge.

Sources : Wikipedia & Google Images