A narrow plain, the Tihamat ash-Sham, parallels the Red Sea coast, as do, farther north, the Hijaz Mountains (with elevations of 910–2,740 m/3,000–9,000 ft), which rise sharply from the sea. The highest mountains (over 2,740 m/9,000 ft) are in 'Asir in the south. 'Asir is a region extending about 370 km (230 mi) along the Red Sea and perhaps 290–320 km (180–200 mi) inland. East of the Hijaz, the slope is more gentle, and the mountains give way to the central uplands (Najd), a large plateau ranging in elevation from about 1,520 m (5,000 ft) in the west to about 610 m (2,000 ft) in the east. The Dahna, a desert with an average width of 56 km (35 mi) and an average altitude of 460 m (1,500 ft), separates Najd from the low plateau (Hasa) to the east (average width, 160 km/100 mi; average altitude, 240 m/800 ft). This, in turn, gives way to the low-lying Gulf region.
At least one-third of the total area is sandy desert. The largest of the deserts is the famed Ar-Rub' al-Khali in the south, with an area of roughly 647,500 sq km (250,000 sq mi). An-Nafud, its northern counterpart, has an area of about 57,000 sq km (22,000 sq mi). There are no lakes, and except for artesian wells in the eastern oases, there is no perennially flowing water.