Collapsed and eroded Sheild Volcano?
I'm sure there's all sorts of possibilities before we get to Atlantis.
Richat Structure - Wikipedia
Initially interpreted as an
asteroid impact structure because of its high degree of circularity, the Richat Structure is now regarded by geologists as a highly symmetrical and deeply eroded
geologic dome. After extensive field and laboratory studies, no credible evidence has been found for
shock metamorphism or any type of deformation indicative of a
hypervelocity extraterrestrial impact.
[1] While
coesite, an indicator of shock metamorphism, had initially been reported as being present in rock samples collected from the Richat Structure, further analysis of rock samples concluded that
barite had been misidentified as coesite.
[2] In addition, the Richat Structure lacks the annular depression that characterizes large extraterrestrial impact structures of this size. Also, it is quite different from large extraterrestrial impact structures in that the sedimentary strata comprising this structure is remarkably intact and "orderly" and lacking in overturned, steeply dipping strata or disoriented blocks.
[3][4] A more recent multianalytical study on the Richat megabreccias concluded that carbonates within the silica-rich megabreccias were created by low-temperature hydrothermal waters, and that the structure requires special protection and further investigation of its origin.
[5]
pick your own theory, it could just be a natural formation that was modified to become an ancient civ. nobody says that it has to be man made. if you look at google earth you can see a direct line from the ocean, so maybe there was an ancient tsunami that washed everything out.