Chott el Djerid is the Sahara's largest salt pan, 250 kilometres of cracked crust in southern Tunisia, and most of the year it does a convincing impression of water. One road crosses it, from Tozeur to Kebili, with nothing but salt either side for kilometres. Somewhere in the middle the shoreline goes out of sight and there is no edge left to anything. When the surface gets hot enough, the air bends light into false water, islands and silhouettes that hang on the horizon and retreat as you drive at them. That works best in the flat glare of midday. At the lay-bys the water is real but strange. Shallow brine pools sit a few steps off the road, pink and turquoise from the minerals and algae in them, strongest at the rims where the crystals are growing.
Winter turns the whole thing over. Rain off the hills to the north spreads a sheet of water across the salt, rarely deeper than your ankles. On a still morning the pan reflects the sky, and the few posts breaking the surface are all that tells you which way is up. Greater flamingos arrive with it and feed far out on the flooded flats. The roadside stalls sell desert roses, which are neither flowers nor fossils. Gypsum crystallises underground in damp, salty sand, trapping grains as it splays out petal on petal, so every one was dug up, not picked up. Twenty minutes off the causeway stands a white dome left on the salt by the first Star Wars, and the walk is the point. The road noise drops away, the crust crunches under you, and the shell grows out of the glare with nothing else in it. People have tried to make the illusion real more than once. In the 1870s a French officer proposed cutting a canal from the sea to flood these pans into an inland one. Jules Verne wrote a novel about it. The scheme keeps running into the same arithmetic: the basin lies above the sea, so the water would have to be lifted, not let in.