White Sands National Monument is a United States national monument located in the state of New Mexico on the north side of Route 70 about 16 miles (26 km) southwest of Alamogordo in western Otero County and northeastern Doña Ana County. The monument is situated at an elevation of 4,235 feet (1,291 m) in the mountain-ringed Tularosa Basin and comprises the southern part of a 275 sq mi (710 km2) field of white sand dunes composed of gypsum crystals. The gypsum dune field is the largest of its kind on Earth.

The idea of creating a national park to protect the white sands formation dates to 1898 when a group from El Paso proposed the creation of Mescalero National Park. The plan called for a game hunting preserve, however, which conflicted with the idea of preservation held by the Department of the Interior, and the plan failed. In 1921–22, Albert Bacon Fall, United States Secretary of the Interior and owner of a large ranch in Three Rivers northeast of the dune field, promoted the idea of an “all-year national park” that, unlike more northerly parks, would be open even in the winter. This idea ran into a number of difficulties and did not succeed. Tom Charles, an Alamogordo insurance agent and civic booster, was influenced by Fall’s ideas. By emphasizing the economic benefits, Charles was able to mobilize enough support to have the national monument created.

Gypsum rarely occurs as sand because it is water-soluble. Rain usually dissolves gypsum and rivers then carry it to the sea. The Tularosa Basin has no outlet to the sea, so it traps rain that dissolves gypsum from the surrounding San Andres and Sacramento Mountains. The rainwater either sinks into the ground, or forms shallow pools that subsequently dry out and leave gypsum on the surface in a crystalline form called selenite. Groundwater that flows out of the Tularosa Basin flows south into the Hueco Basin. During the last ice age, a lake now called Lake Otero covered much of the basin. When it dried out, a large flat area of selenite crystals remained, which is named the Alkali Flat. Lake Lucero, a dry lake bed which occasionally fills with water, is located in the southwest corner of the park, at one of the lowest points of the basin.

The ground in the Alkali Flat and along Lake Lucero’s shore is covered with selenite crystals that measure up to three feet (1 m). Weathering and erosion eventually break the crystals into sand-size grains that are carried away by the prevailing winds from the southwest, forming the 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, which increases the wind resistance of the dunes. The increased 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.