In the middle of the Adrar plateau, in north-western Mauritania, a giant circle spreads across the sand as if someone had drawn it with a compass. This is the Richat Structure, the Eye of the Sahara, one of the strangest geological formations on Earth. It puzzled scientists for a long time: it was taken for a meteorite crater before turning out to be something slower and stranger. A dome of rock rose, then wind and water wore it down over millions of years, uncovering its layers one after another like the rings of a tree trunk. What you walk on here is the inside of the Earth turned up to the sky. From the ground the circle is invisible — too wide for the eye to hold, it appears only from the air or from high ground. What you see instead is ring of stone after ring of stone, and rock old enough to make the desert itself look new.