Tafassasset Wadi whose run, that today gets lost in the sands of the Teneré, once time was reaching the depression of the Ciad lake.

A common mirage is the appearance of an isolated lake frequently seen in hot sandy. As the ground is heated by the hot sun, the air nearby expands in the heat. As it expands, its density becomes lower and its refractive index also becomes lower. Higher up where the air is cooler the density and refraction normalizes in a higher layer. The vast difference in temperatures and density between the two layers can cause a mirror effect to occur. The sky appears as a shining lake; mountains or palms may be similarly reflected, but the images are inverted.