The Ennedi is an island of sandstone the size of Switzerland standing in the Sahara of northeastern Chad, and it holds the only permanent water for hundreds of kilometres. That water is the Guelta d'Archei, a pool at the bottom of a canyon between cliffs hundreds of metres high. Every day the camel herds come down the gorge to drink, hundreds at a time in the shade of the walls. In the black water live the last crocodiles of the Sahara. They stay alive on the camels: the dung blackens the pool, the muck feeds algae, and the algae feed the fish the crocodiles eat. The rim above is where you watch from, and first light is the window, before the herds arrive. You are looking down several hundred metres of cliff, so binoculars do more here than any lens.
At Oyo the rock breaks into a labyrinth of blocks in bare golden sand, deep enough inside that the horizon disappears and the towers close around your camp. Elsewhere the same rock is cut into gorges two hundred metres deep that squeeze to shoulder-width, the sky a bright ribbon overhead. The sandstone does not wear evenly, and the plateau has come out a field of top-heavy pillars, mushroom rocks and arches. The Aloba is the giant among them, and from a distance it reads as one more fin until you stand under the span with the desert framed through the gap. The walls carry a gallery of a greener Sahara, and the animals change down the centuries: wild giraffes and elephants first, then herded cattle, and camels only at the end. At Niola Doa, the place of the girls, engraved figures stand life-size or larger, hips and thighs enormously wide, bodies filled with pecked squares and wavy lines that may be clothing or tattoos. The crocodiles are the other leftover. They are the same species that fills West African rivers, stunted here and down to a handful, and nothing has come in or gone out since the last rivers dried.