The White Sands National Monument is a U.S. National Monument located about 25 km (16 mi) southwest of Alamogordo in western Otero County and northeastern Doña Ana County in the state of New Mexico, at an elevation of 4235 feet (1291 m). The area is in the mountain-ringed Tularosa Basin and comprises the southern part of a 710-km² (275-mi²) field of white sand dunes composed of gypsum crystals. It is the largest gypsum dune field in the world.
Gypsum is rarely found in the form of sand because it is water-soluble. Normally, rain would dissolve the gypsum and carry it to the sea. The Tularosa Basin is enclosed, meaning that it has no outlet to the sea and that rain that dissolves gypsum from the surrounding San Andres and Sacramento Mountains is trapped within the basin. Thus water either sinks into the ground or forms shallow pools which subsequently dry out and leave gypsum in a crystalline form, called selenite, on the surface. Groundwater that does flow out of the Tularosa Basin flows south into the Hueco Basin. During the last ice age, a lake known as Lake Otero covered much of the basin. When it dried out, it left a large flat area of selenite crystals which is now the Alkali Flat. Another lake, Lake Lucero, at the southwest corner of the park, is a dry lake bed, at one of the lowest points of the basin, which occasionally fills with water.
The ground in the Alkali Flat and along Lake Lucero's shore is covered with selenite crystals which reach lengths of up to three feet (1 m). Weathering and erosion eventually breaks the crystals into sand-size grains that are carried away by the prevailing winds from the southwest, forming white dunes. The dunes constantly change shape and slowly move downwind. Since gypsum is water-soluble, the sand that composes the dunes may dissolve and cement together after rain, forming a layer of sand that is more solid and could affect wind resistance of dunes. This resistance does not prevent dunes from quickly covering the plants in their path. Some species of plants, however, can grow fast enough to avoid being buried by the dunes.
Various forms of dunes are found within the limits of White Sands. Dome dunes are found along the southwest margins of the field, transverse and barchan in the core of the field, and parabolic dunes occur in high numbers along the northern, southern, and northeastern margins. From the visitor center at the entrance of the park, the Dunes Drive leads 8 miles (13 km) into the dunes. Four marked trails allow one to explore the dunes by foot. During the summer, there are also Ranger-guided orientation and nature walks. The park participates in the Junior Ranger Program, with various age-group-specific activities .
Unlike dunes made of quartz-based sand crystals, the gypsum does not readily convert the sun's energy into heat and thus can be walked upon safely with bare feet, even in the hottest summer months. In areas accessible by car, children frequently use the dunes for downhill sledding. Because the park lies completely within the White Sands Missile Range, both the park and U.S. Route 70 between Las Cruces, New Mexico and Alamogordo are subject to closure for safety reasons when tests are conducted on the missile range. On average, tests occur about twice a week, for a duration of one to two hours. Located on the northernmost boundaries of White Sands Missile Range, the Trinity Site can be found, where the first atom bomb was detonated.