Because the country's southern borders with the United Arab Emirates and Oman are not precisely defined or marked, the exact size of the country remains unknown. The CIA World Factbook's estimate is 2,149,690 km2 (830,000 sq mi) and lists Saudi Arabia as the world's 13th largest state.
Saudi Arabia's geography is dominated by the Arabian Desert and associated semi-desert and shrubland . It is, in fact, a number of linked deserts and includes the 250,000 square mile (647,500 square km) Rub' al Khali (“Empty Quarter”) in the southern part of the country, the world’s largest sand desert. There are virtually no permanent rivers or lakes in the country, but wadis are numerous. The few 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 Arabian 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 Mount Sawda, which is generally considered the highest point in the country. Estimates of its elevation range from 10,279 to 10,522 feet (3,133 to 3,207 metres).