21.1240°N, 11.4014°W.
Concentric ring structure in the Adrar Plateau of Mauritania, exposed in the Sahara as a 40-km-diameter bullseye of sedimentary and igneous rocks. Originally identified as a meteor impact crater; subsequent isotopic and stratigraphic work (Matton, Gaboury, Schmitt 2005, Matton & Jébrak 2014) reclassified it as a deeply eroded magmatic dome — concentric rings expose successive layers of Cambrian to Ordovician sediment, intruded by 100-million-year-old kimberlite and carbonatite. The structure is best seen from orbit; before satellite imagery in the 1960s, the rings were unknown to mainstream geology because no observation point on the surface is high enough to perceive the pattern. The center contains a breccia of mixed rock types and a hydrothermal carbonate deposit.
A bullseye visible only from space sat in the Sahara unrecognized until astronauts looked back at Earth.