Saudi Arabia is the largest country in the world that does not have a single permanent river. Instead, it relies on 'wadis' (dry riverbeds that only flow during rare rainstorms), groundwater from deep aquifers, and the world's largest desalination plants to provide water for its population. Despite being nearly 1/3 the size of the United States, its hyper-arid climate and vast deserts (including the Rub' al Khali, or Empty Quarter) prevent the formation of permanent surface water. To sustain its cities and agriculture, Saudi Arabia has become a global leader in water technology, turning salt water into fresh water on a massive scale.