Rub' al Khali, or The Empty Quarter, is the largest sand desert in the world. It covers 650,000 square kilometres (251,000 square miles), and includes parts of Saudi Arabia, Oman, Yemen, and the United Arab Emirates. In the centre of the desert there are a number of raised, hardened formations that were once the sites of shallow lakes, thousands of years ago. For a sense of scale, this Overview shows approximately 350 square kilometres (135 square miles) in Saudi Arabia, near the border with Oman.