It's a water-collecting basin with evidence that rivers flowed from it at some point. I guess when the area was lush, 12000 years ago, but you can see it on maps as recently as 500 BC showing rivers leading to that general area.
What's interesting to me is the fact that it's on a hill combined with the fact that a large amount of that area was very low in elevation (to the point that there are plenty of theories that a lot of western Africa was actually submerged in the past). So there might have been a situation where the Richat Structure and its hill actually existed as an island (ala Plato's version) with the Atlantic significantly further inland.
Who knows, though. The main issue with the Atlantis theory is, indeed, where the hell the pieces of it went. My best guess is the Flood either dragged all of it back into the ocean, or scattered all of it across what's now the Sahara, to be buried forever by the onset of the sand. Probably a whole lot of settlements under that desert, considering rivers and lakes used to flow right through all of it for tens of thousands of years before the sand arrived.
