Rub al Khali is a great desert of the Arabian peninsula, one of the largest sand deserts in the world. The desert occupies much of the southern interior of the peninsula, from the highlands of the north to the plateaus of the south. Sand dunes rise to over 200 in the southwest; there are salt marshes and pans in the southeast. The desert comprises more than 25% of Saudi Arabia. It is extremely dry and virtually uninhabited.