They may look like mounds of snow, but the dunes in this park are made of a rare form of crystallized gypsum. Because gypsum dissolves easily in water, rain would normally wash it away and carry it to the sea. In this part of the Chihuahuan Desert, however, the land forms a basin, trapping the mineral; water evaporates, leaving the gypsum behind, and wind and weather erode it over time into an ocean of glittering sand. The entire dune field is a massive 275 square miles (by comparison, the second-largest gypsum dune field in the world, Cuatro Ciénegas in Mexico, is only 8 square miles). Hiking and sledding over this vast white expanse of powder is a singular, otherworldly experience.