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.