The Rub' al Khali (Arabic: الربع الخالي), which translates as Empty Quarter in English, 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 kilometers (250,000 square miles) (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 and almost the land area of Texas.
Largely unexplored until recently, the desert is one thousand kilometers (600 miles) long, and 500 km (300 mi) wide. Even the Bedouins only skirt the edges of the desert. Nonetheless, tour companies do exist that offer GPS-equipped excursions into the desert. The first documented journeys made by Westerners to the Empty Quarter were those made by Bertram Thomas in 1931 and St. John Philby in 1932. Between 1946 and 1950 Wilfred Thesiger crossed the area several times and mapped large parts of the Empty Quarter and the mountains of Oman.
With summer temperatures up to nearly 55 degrees Celsius (131 F) at noon, and dunes taller than the Eiffel Tower — over 330 meters (1000 ft) — the desert may be the most foreboding environment on Earth. However, as nearly everywhere else, life flourishes. Arachnids, rodents and plant life can all be found throughout the Empty Quarter. As an ecoregion, it falls within the Arabian Desert and East Sahero-Arabian xeric shrublands.
Desertification has increased through the millennia. Before desertification made the caravan trails leading across the Rub' al Khali so difficult, the caravans of the frankincense trade crossed now virtually impassable stretches of wasteland, until about AD 300. For example, Iram of the Pillars, a lost city, depended on such trade. More recently, tribal populations were also present in certain parts of the Empty Quarter, with the largest in the Najran region. A few road links were connected with these tribal settlements to the water resource and oil production centers.
Geologically, the Empty Quarter is one of the most oil-rich places in the world. Vast oil reserves have been discovered underneath the sand stacks. 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.