The Rub' al Khali or Empty Quarter is one of the largest sand deserts in the world, encompassing most of the southern third of the Arabian Peninsula, including most of Saudi Arabia and areas of Oman, the United Arab Emirates, and Yemen. The desert covers some 650,000 square kilometres (250,000 sq mi).
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 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.
Along the middle length of the desert there are a number of raised, hardened areas of calcium carbonate, gypsum, marl, or clay that were once the site of shallow lakes. These lakes existed during periods from 37,000 to 17,000 years ago and 10,000 to 5,000 years ago. The lakes are thought to have formed as a result of "cataclysmic rainfall" similar to present-day monsoon rains and most probably lasted for only a few years. However, lakes in the Mundafen area in the southwest of the Rub' al Khali show evidence of lasting longer, up to 800 years, due to increased runoff from the Tuwaiq Escarpment.