sea. The trapped water sank into the ground or formed shallow pools that subsequently dried out, leaving the 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 body of water named 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 is a dry lakebed in the southwest corner of the park, at one of the lowest points in the basin. Rain and snowmelt from the surrounding mountains and upwelling from deep groundwater within the basin periodically fill Lake Lucero with water containing dissolved gypsum. When filled, the lake covers about at a depth of . As the water evaporates, small selenite crystals about in diameter form on the surface of the lake. Most of the crystal formation occurs when large floods concentrate the mineralized water every ten to fourteen years. Wind and water break down the crystals into progressively smaller particles until they are fine grains of white gypsum sand. The ground in the Alkali Flat and along Lake Lucero's shore is also covered with selenite crystals that measure up to long. 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 can grow fast enough to avoid being buried by the dunes, while others utilize survival strategies such as the formation of a hardened pedestal around their roots to stabilize the plant amid the moving dunes. White Sands contains various forms of dunes. Dome dunes are found along the southwest margins of the field, transverse and barchan dunes in the center of the field, and parabolic dunes along the northern, southern, and northeastern margins. Scientists have surveyed a long strip of dunes with lasers; installed shallow water wells across the dunefield; looked at the chemistry and age of the water below the dunes;