Durable dunes

Most of the United Arab Emirates' 11 million-plus people live in the urban cluster at the nation's northeast corner, including global cities like Dubai and Abu Dhabi. But here we see a much sleepier sector: The Rub' al Khali, or Empty Quarter. The world's largest erg, the quarter covers 250,000 square miles of the lower Arabian Peninsula with countless dunes, many reaching over 800 feet tall. Packed tight and soaked deep down with groundwater, these mountains of sand tend to keep their shape in spite of the fierce desert winds that whip their surface.