Imagine the desert or a wide beach—an endless sea of sand. The wind blows, picking up grains of sand and sending them whirling through the air. When the wind slows down or stops, the sand falls to the ground in wavelike patterns. Ripples and hills form. The hills are called dunes. In some areas, wind erosion causes the dunes to move from place to place. In other areas, the water below the surface of the sand helps the dunes keep most of their original size and shape.