Yemen is in the southern half of the Arabian Peninsula, bordering the Red Sea, the Gulf of Aden and the Arabian Sea. The country is divided into four geographical regions: the Tihamah or coastal plains to the west, the western highlands, the central highlands, and the Rub' al Khali, or Empty Quarter, in the east, the largest sand desert in the world. The Tihamah forms an arid flat plain alongside the Red Sea coast. There are many lagoons here and considerable biodiversity; streams from the western highlands sink and evaporate before reaching the coast.