Saudi Arabia's diverse geography is dominated by the Arabian Desert, associated semi-desert, shrubland, steppes, several mountain ranges, volcanic lava fields and highlands. The  Rub' al Khali ("Empty Quarter") in the southeastern part of the country is the world's largest contiguous sand desert. Though there are lakes in the country, Saudi Arabia is the largest country in the world by area with no permanent rivers. Wadis, non-permanent rivers, however, are very numerous. The fertile areas are to be found in the alluvial deposits in wadis, basins, and oases. The main topographical feature is the central plateau which rises abruptly from the Red Sea and gradually descends into the Nejd and toward the Persian Gulf. On the Red Sea coast, there is a narrow coastal plain, known as the Tihamah parallel to which runs an imposing escarpment. The southwest province of Asir is mountainous, and contains the  Mount Sawda, which is the highest point in the country.