The Eye of the Sahara, also known as the Richat structure, is situated in Mauritania in the Western Sahara Desert. The formation is around 25 miles across. Because of its size and remarkable shape, astronauts use the Eye as a key landmark. The origin the structure is still a topic of lively debate.
For quite some time scientists believed that the Eye of the Sahara was an impact crater. But they didn't find enough melted rock to make that guess hold water. Current theories suggest a much more complicated story behind this incredible natural formation.
The main ring structure of the Eye is the eroded remains of what was once a dome of layers of Earth's crust. Scientists still have questions about the Eye of the Sahara, but two Canadian geologists have a working theory about its origins. They think that the Eye's formation began more than 100 million years ago, as the supercontinent Pangaea was ripped apart by plate tectonics and what are now Africa and South America were being torn away from each other.
Molten rock pushed up toward the surface but didn't make it all the way, creating a dome of rock layers, like a very large pimple. A little after 100 million years ago, the Eye erupted violently. That collapsed the bubble partway, and erosion did the rest of the work to create the Eye of the Sahara that we know today.
There are some people who think that the Eye of the Sahara might actually be the remains of the city Atlantis, this is amongst other things based on the fact that Plato has described Atlantis as a city with concentric rings of land and water. (Please don't take this too seriously, although the fact that there have been civilizations living at the location of the current Sahara a few millennia ago is likely)