Eye of the Sahara
The Eye of the Sahara, also known as the Richat Structure, forms a bulls-eye of sorts in the middle of an otherwise featureless desert area near Quadane, Mauritania. The structure is about 30 miles in diameter, according to NASA — it's so big and colorful that astronauts use it as a landmark from space.
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." Today, the "uplift" stands about 650 feet high — nearly 1,600 feet above sea level.
Why is it blue? 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.