White Sands National Monument is a U.S. National Monument located in the state of New Mexico on the north side of Route 70 about 16 miles southwest of Alamogordo in western Otero County and northeastern Doña Ana County. The monument is situated at an elevation of 4,235 feet in the mountain-ringed Tularosa Basin and comprises the southern part of a 275 sq mi field of white sand dunes composed of gypsum crystals. The gypsum dune field is the largest of its kind in the world.