NASA satellite images of the Richat Structure.
About the Richat Structure:
The Richat Structure, also known as the Eye of the Sahara and Guelb er Richat, is a prominent circular feature in the Sahara desert of west–central Mauritania near Ouadane. This structure is a deeply eroded, slightly elliptical, 40-km in diameter, dome. The sedimentary rock exposed in this dome range in age from Late Proterozoic within the center of the dome to Ordovician sandstone around its edges.