The Lakes of Ounianga are eighteen lakes coloured blue, green and crimson in the Borkou region of northern Chad, in the driest part of the Sahara, and they live on water that fell as rain ten thousand years ago, when the desert was a savanna, seeping in from under the ground. The lakes lie in two groups in a basin between sandstone cliffs and dunes, Ounianga Kébir with the big lakes, and Ounianga Sérir with a cluster of smaller ones strung between the dunes, each ringed by a thin band of date palms and reeds, and they are fed by a fossil aquifer, the rain of the wetter Sahara stored in the rock, which is why they persist where every other lake in the desert dried; some are fresh and some so salty that the water holds a bather on the surface without effort, as in the Dead Sea, and some glow a vivid green from spirulina, the alga sold as a superfood, blooming wild in water too salty for almost anything else. Lake Yoa, the largest and deepest, is blue, and on its far shore a lone rock pyramid stands doubled in the water, sharpest in the calm before the wind gets up.
The floor of Lake Yoa is a calendar. Drilled, the mud comes up finely banded, one thin layer for every year, a record that scientists count like tree rings to read how the Sahara dried over six thousand years. People herded and settled around the lakes while the country was green, and as the water and grass drew back they drifted away; today two small villages remain beside the lakes, with gardens in the palm groves and a track to Faya-Largeau. The lakes are a UNESCO site and among the most remote on the list, reached by expedition across the desert from N'Djamena over many days. Ounianga is water in the driest desert on Earth, and the rain that made it stopped falling before the pyramids.