The Rub' al Khali (Arabic: الربع الخالي) or Empty Quarter is one of the largest sand deserts in the world, encompassing most of the southern third of the Arabian Peninsula, including southern 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 area between long. 44°30′ −56°30′E., and lat. 16°30′ −23°00′N), more than the combined land areas of the Netherlands, Belgium, and France. The desert is neither inhabited nor traversed by the Bedouin
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.
Evidence suggests that the lakes were home to a variety of flora and fauna. Fossil remains indicate the presence of several animal species, such as hippopotamus, water buffalo, and long-horned cattle. The lakes also contained small snails, ostracods, and when conditions were suitable, freshwater clams. Deposits of calcium carbonate and opal phytoliths indicate the presence of plants and algae. There is also evidence of human activity dating from 10,000 to 5,000 years ago, including chipped flint tools, but no actual human remains have been found.
The region is classified as "hyper-arid", with typical annual rainfall of less than 35 millimetres (1.4 in). Daily maximum temperatures average at 47 °C (117 °F) during July and August and can reach temperatures as high as 51 °C (124 °F).
Fauna includes arachnids and rodents while plants live throughout the Empty Quarter. As an ecoregion, the Rub' al Khali falls within the Arabian Desert and East Saharo-Arabian xeric shrublands.
Geologically, the Empty Quarter is the second most oil-rich place in the world. Vast oil reserves have been discovered underneath the sand dunes. Sheyba, in the middle of the desert, is a major Arab light crude oil-producing site in Saudi Arabia. Also, Ghawwar Field, the largest oil field in the world, extends southward into the northernmost parts of the Empty Quarter.