What is the world's largest sand desert (erg)?

The Rub' al Khali is the world's largest sand desert, or erg.

Also called the Empty Quarter, it occupies much of the southern Arabian Peninsula and covers approximately 650,000 square kilometres. Most of its surface consists of vast sand seas and dunes, although gravel plains, gypsum deposits, salt flats, and ancient lake beds also occur across the region.

The desert extends through Saudi Arabia, Oman, the United Arab Emirates, and Yemen. Some dunes rise to around 250 metres, and the sand often appears reddish-orange because it contains feldspar. The name Rub' al Khali means “Empty Quarter,” reflecting the region's extreme aridity and sparse population.

The Sahara is much larger overall, but it is not a single continuous sand desert: large parts are rocky or gravel-covered. The Taklamakan and Gobi are also major deserts, yet neither is the world’s largest erg. Annual rainfall in the Rub' al Khali is generally below 50 millimetres, making it one of Earth’s hyper-arid environments.