The Lakes of Ounianga are eighteen lakes in the driest part of the Sahara, in the Borkou region of northern Chad, and they are drinking rain that fell about ten thousand years ago. No river reaches them. They live on fossil groundwater, the rain of a wetter Sahara stored in the sandstone, and they are still here when every other lake in this desert has dried out. They lie in two clusters: the big lakes at Ounianga Kébir, and eastward at Ounianga Sérir a string of smaller ones between the dunes, each ringed by date palms and reeds. The colours are a readout of salt. Some are blue; some glow vivid green with spirulina, the alga sold as a superfood, blooming wild in water too salty for almost anything else; the salt-reddened ones are crimson.
Lake Yoa is the one people wade into. Its water carries so much salt that you bob on the surface without effort, the way you would in the Dead Sea. You wade in from a palm-shaded shore, with bare dunes climbing straight out of the water behind you. On the far side a lone rock pyramid stands doubled in the lake, sharpest before the wind gets up. As the sun drops, the sandstone walls behind it run from white through amber to deep rose. The reflection goes first: once the surface ruffles, the doubled pyramid breaks apart and the colour stays only on the rock. At Sérir sandy paths link one small lake to the next, and where a dune slides down into the water the border between Sahara and oasis narrows to a few steps. The floor of Lake Yoa holds the record of how this ended: drilled, the mud comes up finely banded, one thin layer for every year, and a team counted about 10,940 of them. People herded and settled here while the basin was green, and as the water and the grass drew back they drifted away. Two small villages stayed, with gardens in the palm groves, beside water that has not been renewed since.