An hour before dawn, in a nameless rock pile in the world’s largest hot desert, Djimet Guemona clambered up a narrow gully. The route was hemmed by high walls, where uncountable centuries of weathering had cinched the sandstone into a rumpled, reptilian skin. From its base, the outcropping had looked like any one of the numerous rock formations, or hoodoos, in the northwest corner of the Ennedi Natural and Cultural Reserve, in eastern Chad.