The Richat Structure (Eye of the Sahara)

The Richat Structure, in the Sahara desert of west-central Mauritania, is a roughly 40-kilometre-wide series of concentric rings of differentially eroded rock, so vast and symmetrical that it was first widely noticed not from the ground but from space, by astronauts during early crewed orbital missions in the 1960s, who used it as a landmark. For decades it was suspected to be an impact crater given its circular form, but geological surveys have found no evidence of shock metamorphism or other impact-related features; it is now understood to be the result of a symmetrical dome of uplifted rock that has been eroded over millions of years, with alternating layers of harder and softer rock eroding at different rates to produce the concentric ring pattern.

The structure's striking resemblance to Plato's description, in his dialogues Timaeus and Critias, of Atlantis as a city built on a series of concentric rings of land and water, has made it a recurring candidate among Atlantis researchers since at least the 1990s — proponents point to the structure's size and concentric form as matching Plato's measurements more closely than most other proposed locations. Mainstream geologists and archaeologists note that the Richat Structure is a purely geological formation with no evidence of artificial construction, walls, canals, or any of the architectural features Plato describes, and that the region was likely arid desert rather than the fertile plain Plato's account requires for the entire span of human history — making the identification, in the assessment of specialists, a case of visual pattern-matching rather than archaeological evidence.