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.

But this one, Guemona assured me, contained a special treasure. Though the formation’s interior was labyrinthine, the archaeologist moved swiftly, ignoring the GPS coordinates stored on his phone. He had the location committed to memory.

Rounding a scarp, Guemona stopped in front of a rare expanse of vertical plane where the rock ran smooth. Under torchlight, I could see that hundreds of figures had been carved into its surface. There were ostriches with lollipop heads beside stick-legged giraffes, their necks comically exaggerated. Human forms stood among this crowded bestiary, and beneath them all, in palimpsest, were larger etchings of elephants, older and more deeply inscribed.

“When I found it, I yelled with excitement,” Guemona told me, casting his headlamp left and right so that the shapes appeared to dance across the rock face. These were prehistoric etchings that hadn’t been mentioned in any of the colonial literature he’d studied. Nor had Guemona received any rumor of their existence from the herders whom he often canvassed for information. Whoever carved them inhabited a deeper past, perhaps 10,000 years ago, when the creatures they depict­ed could still be found throughout North Africa. These images were ghosts from a time when the Sahara was green.