There are large rivers to the east and west of the basin that carry rainwater to the Gulf of Mexico, but all the rainwater that falls inside the basin stays here. It collects at the lowest spot on the basin floor. This spot is Lake Lucero, and it's located at the southwest corner of the dune field. It is here at Lake Lucero that the sand dunes begin to form. Lake Lucero is a playa, or seasonal lake.
Water only collects there during the rainy season, and the rest of the year is it dry.
Even when the lake has water, it is only about 12 inches deep, so it is not a place you could go fishing, or swimming. As the water begins to evaporate, the gypsum that is in the water begins to form a crust on the lake bed. Some of the gypsum forms beautiful crystals called selenite. Selenite crystals are very soft. You can scratch them with your fingernail.