Rub' al Khali

Rub' al Khali, also called the Empty Quarter, spans most of the southern third of the Arabian Peninsula across Saudi Arabia, Oman, the United Arab Emirates and Yemen, covering about 650,000 square kilometers over roughly 1,000 kilometers in length and 500 kilometers in width, making it one of the largest continuous sand seas, or ergs, on Earth. Its dunes reach up to 250 meters high and are interspersed with gravel and gypsum plains, with the sand's reddish-orange color coming from feldspar content; surface elevation ranges from about 800 meters in the southwest down to sea level in the northeast, and the Umm al Samim area holds brackish salt flats. Beneath the dunes, evidence of ancient lake beds points to two wetter periods, one roughly 6,000 to 5,000 years ago and another roughly 3,000 to 2,000 years ago, when episodic rainfall and runoff from the Tuwaiq Escarpment fed lakes that sometimes persisted for up to 800 years; fossils of hippopotamus, water buffalo and long-horned cattle from those lakes have been found preserved in the desert. This entity documents Rub' al Khali specifically as a geological formation, an aeolian sand sea that has periodically held ancient lakes over the last several thousand years.