evidence: Saudi Arabia's geography is dominated by the Arabian Desert, associated semi-desert, shrubland, steppes, several mountain ranges, volcanic lava fields and highlands. The 647500 km2 (250001 sq mi) Rub' al Khali ("Empty Quarter") in the southeastern part of the country is the world's largest contiguous sand desert.[140][141] Though there are lakes in the country, Saudi Arabia is the largest country in the world by area with no permanent rivers. Wadis, non-permanent rivers, however, are very numerous throughout the kingdom. The fertile areas are to be found in the alluvial deposits in wadis, basins, and oases.[140] There are approximately 1,300 islands in the Red Sea and Arabian Gulf.[142]