Eye of the Sahara (Guell Er Richat).
Richat is a geological formation located in the Mauritanian part of the Sahara Desert near the settlement of Ouadan. The diameter of the structure is 50 km. It is believed that Richat was formed between the late Proterozoic and Ordovician (the oldest ring is 500-600 million years old). Previously, it was assumed that the ring structure is an impact crater, but this hypothesis is not consistent with the flat shape of the bottom and the complete absence of rocks with impact traces. The formation of the structure as a result of a volcanic eruption is also extremely unlikely due to the lack of a dome of igneous or volcanic rocks. Clearly it was an explosion. From uneven surfaces, you can determine that there were buildings, possibly a city. And now there was only a "burn" on the ground.