The Rub' al Khali desert is the largest contiguous sand desert (erg) in the world, encompassing most of the southern third of the Arabian Peninsula. The desert covers some 650,000 km2 (250,000 sq mi) (the area of long. 44°30′−56°30′E, and lat. 16°30′−23°00′N) including parts of Saudi Arabia, Oman, the United Arab Emirates, and Yemen. It is part of the larger Arabian Desert.
The desert is 1,000 kilometres (620 mi) long, and 500 kilometres (310 mi) wide. Its surface elevation varies from 800 metres (2,600 ft) in the southwest to around sea level in the northeast. The terrain is covered with sand dunes with heights up to 250 metres (820 ft), interspersed with gravel and gypsum plains. The sand is of a reddish-orange color due to the presence of feldspar.
There are also brackish salt flats in some areas, such as the Umm al Samim area on the desert's eastern edge.