Rub' al Khali
The Rub' al Khali (Arabic: الربع الخالي ar-Rubʿ al-Ḫālī, Empty Quarter) is the largest contiguous sand desert in the world, encompassing most of the southern third of the Arabian Peninsula. It is the cradle of civilization on Earth, containing the ruins of Ubar, an ancient city that predates human civilizations by at least 4,000 years. 
Rub' al Khali is mentioned in Fallout 4.
- Jack Cabot: "My father excavated a city in the Rub'al Khali in Arabia which he dated to more than 4,000 years before the rise of any known human civilization. The structures and artifacts were... strange. Disturbing, even. Clearly not constructed for or by humans. I've spent my life trying to decipher what he uncovered."
- Jack Cabot: "Millennia older than the earliest human civilizations. But with technology that seems to have surpassed our own. And yet, everything about it is... strange. Disturbing geometries, tools not made for human hands, carvings that hint at dimensions beyond our own..."
- Jack Cabot: "He believed that all of the earliest human civilizations shared some common parent that was unknown to history. Think of Atlantis - the myth of an advanced civilization destroyed by a cataclysm is shared by many widely separated ancient cultures. Eventually, he found was he was looking for - a lost city, buried beneath the sands of the Empty Quarter of the Arabian desert."
- Jack Cabot: "Yes. He spent years looking, and was widely ridiculed by the so-called experts. I'm ashamed to say even I doubted him. But in the end he found it: a lost city, buried in the sands of the Rub'al Khali, the Empty Quarter of Arabia."
- Jack Cabot: "You wouldn't have. My father only made one expedition to the lost city, and his findings were never published. The clues were all there, scattered amongst the earliest civilizations... but he was the only one to understand what they meant. He followed them into the Empty Quarter of Arabia, to the nameless city buried in the sand like the mythical Atlantis beneath the waves..."
- Cabot House terminals: "I've lately immersed myself in Lorenzo's work like never before. I wish now I had made more effort (when it was still possible) to try to locate the site of so-called Ubar. After my initial failed attempt to find the site, I dismissed it as both hopeless and useless - better to focus my energy on the immediate problems. But now, after going over Lorenzo's papers more thoroughly, I believe there are enough clues to the general location that a large-scale search using aircraft might have been feasible - something that wasn't possible in my first attempt, of course. Maybe I could have found some other artifacts that I could have studied more directly - even if they were for completely different purposes, it would be invaluable in understanding how the ancient technology functioned.
I've also come to realize how brilliant Lorenzo actually was. I was far too quick to dismiss his theories - ah, the arrogance of youth! I'm now convinced that his essential premise is correct - the artifact he found was not made by humans. All the evidence was there, but Lorenzo was the only one who put it all together and actually understood what it meant. A staggering thing to think of - if only he had been capable of dealing with fools a bit more diplomatically, he might have gained more general acceptance of his theories and not been driven to such secrecy in his final expeditions."