#6. Richat Structure, Mauritania
Seemingly swirling and spinning and twisting like a cyclone through the heart of the mighty Sahara Desert, the great Richat Structure in the depths of Mauritania is something truly mysterious . Scientists have puzzled for years as to how the perfect circular set of concentric rings got there. Some think it was an asteroid impact in centuries gone by. Others say it was the simple process of natural geological attrition and erosion.There are some who think it was the creation of extra-terrestrials, who passed this way and marked a landing point for future visits to earth.