The largest gypsum dune field in the world is located at White Sands National Monument in south-central New Mexico. This region of glistening white dunes is in the northern end of the Chihuahuan Desert within an "internally drained valley" called the Tularosa Basin. The monument ranges in elevation from 3890' to 4116' above sea level. There are approximately 275 total square miles of dune fields here, with 115 square miles (about 40%) located within White Sands National Monument. The remainder is on military land that is not open to the public. This dune field is very dynamic, with the most active dunes moving to the northeast at a rate of up to 30 feet per year, while the more stable areas of sand move very little. The pure gypsum (hydrous calcium sulfate) that forms these unusual dunes originates in the western portion of the monument from an ephemeral lake or playa with a very high mineral content. As the water evaporates (theoretically as much as 80" per year!), the minerals are left behind to form gypsum deposits that eventually are wind-transported to form these white sand dunes. Many species of plants and animals have developed very specialized means of surviving in this area of cold winters, hot summers, with very little surface water and highly mineralized ground water.