Between the Sierra Nevada and the Colorado River is a vast desert covering 25,000 square miles. Once covered by the Pacific Ocean, it was cut off from the sea by rising mountain ranges. Volcanic activity in the past has covered the area with ash, lava and mud. Today, many small isolated mountain ranges and extinct volcanoes break up the great stretches of sandy soil. Dry lakebeds include Searles and Owens lakes. These lake beds and the region south of them are a principal source of the mineral boron.