The Richat Structure
100 million years (geological); proposed human use unknown
The Richat Structure — also called the Eye of the Sahara — is a 50-kilometre-wide geological formation in Mauritania consisting of perfectly concentric rings visible from space. It was long considered a meteor impact crater, but is now known to be an eroded geological dome. In recent years, researchers have noted that its dimensions, location, and concentric ring structure match Plato's description of Atlantis with remarkable precision.