Kazakhstan's terrain extends west to east from the Caspian Sea to the Altay Mountains and north to south from the plains of Western Siberia to the oases and deserts of Central Asia. The Kazakh Steppe (plain), with an area of around , occupies one-third of the country and is the world's largest dry steppe region. The steppe is characterised by large areas of grasslands and sandy regions. Major seas, lakes and rivers include Lake Balkhash, Lake Zaysan, the Charyn River and gorge, the Ili, Irtysh, Ishim, Ural and Syr Darya rivers, and the Aral Sea until it largely dried up in one of the world's worst environmental disasters.