Two hundred and seventy-five square miles of white sand dunes, the largest gypsum sand dune field in the world. This is the White Sands National Monument just south of Alamogordo, NM. The source of the sand is the gypsum that is trapped in the rock layers of the surrounding mountains. Rain falls on the mountains and the runoff collects on the desert floor. When the water evaporates, the gypsum collects in crystal formations. Wind then takes over and tumbles the crystals until they break down into grains of sand. These are active dunes, moving thirty to forty feet per year. Surprisingly, the dunes have formed in only the past 10,000 years.