Here's a satellite image of the 275 square mile gypsum field of the White Sands National Monument.

At the northern end of the Chihuahuan Desert lies a mountain-ringed valley, the Tularosa Basin. The Basin is lower than the surrounding mountains, and water running into it cannot flow out. The water carries gypsum from the mountains into Lake Lucero, and after the water evaporates, glistening white sand remains. Then, slowly but relentlessly the sand, driven by strong southwestern winds, covers everything in its path--forming the world's largest gypsum dune field.