Entering the Sahara, the rock and hard-packed sand is so flat and immense you can see the Earth’s curve. It’s an optical illusion, really, the mind providing an explanation in the face of such an expanse, the way it interprets the glassy reflections in the near distance as water. On very hot days, the sun bleaches the sky and sand, whiting out all points of reference. It can be literally mind-blowing, inducing Sahara Syndrome, a temporary anxiety disorder that can push you to the fringes of insanity.