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The Empty Quarter of Saudi Arabia, known as Rub' al Khali in Arabic, is one of the least habitable places in the world. It is the largest unbroken expanse of sand in the world, and the Arabian peninsula's largest desert. Taking up much of the lower quarter of the Arabian peninsula, the Empty Quarter has an area larger than France, Belgium, and the Netherlands combined, covering some 650,000 square kilometers (200,000 sq mi). It stretches across the countries of Saudi Arabia, Yemen, Oman, and the United Arab Emirates.
Rarely explored until recently, the Empty Quarter is 1,000 km (620 mi) long and 500 km (310 mi) wide. During summer, temperatures approach 55 degrees C (131 degrees F) at noon. This region is covered with monotonous sand dunes as tall as 330 m (1,100 ft). Many of them are red due to their iron oxide (rust) content. It is qualified as a "hyper-arid" climate, with about 35 mm (1.38 in) of rain per annum, which remarkably is enough to sustain some life. Though the area is hyper-arid, it gets more rain than, for instance, the Atacama Desert in Chile, which is best described as "rainless" and contains absolutely no life.
Though the Empty Quarter does have life, it has very low biodiversity. There are only 37 plant species, with 20 in the main body of the sands and 17 found in the margins. Only one or two are thought to be endemic. This isn't much, but it's better than nothing. Though vegetation is very sparse, it is evenly distributed, found throughout most of the quarter. Animals that make their home in this area include gazelles, oryx (antelopes with straight horns), sand cats (beautiful domestic cat-sized desert wildcats that get all their water from prey animals), spiny-tailed lizards, and many others.
The Empty Quarter is a mysterious locale with many secrets. Underneath its mountainous dunes are vast oil reserves, formed millions of years ago when the area was tropical rainforest. The Empty Quarter was less arid in the past, evidenced by fossils of hippos, water buffalo, and long-horned cattle found there. Camel trains made it across the desert, until it became impassable sometime around 300 CE. A lost city, Iram of the Pillars, is thought to be in the desert, and possibly even an entire lost civilization. A few artifacts have been found using ground-penetrating radar and analysis of satellite imagery, but a lost city remains elusive.