An unlikely tree stands amid the seemingly endless dunes of the Rub" al Khali desert in the Arabian Peninsula. The Rub" al Khali, meaning "Empty Quarter" in Arabic, is a vast sea of sand. Spreading across 250,000 square miles, it"s the largest area of continuous sand in the world, covering most of southeastern Saudi Arabia, with smaller portions in Yemen, Oman, and the United Arab Emirates. One of the driest places on Earth, almost no one lives in the Rub" al Khali, and much of the desert remains unexplored. Some of the dunes tower more than 800 feet, and the depressions in between—called sabkhas—can be so soft that vehicles, camels, and people can easily become stuck.