The Sahara (, , 'the Greatest Desert') is the largest hot desert and third largest desert after Antarctica and the Arctic worldwide. Its surface area of —including the Libyan Desert—is comparable to the respective land areas of China or the United States. The desert comprises much of the land found within North Africa, excluding the fertile coastal region situated against the Mediterranean Sea, the Atlas Mountains of the Maghreb, and the Nile Valley of Egypt and Sudan. The Sahara stretches from the Red Sea in the east and the Mediterranean in the north, to the Atlantic Ocean in the west, where the landscape gradually transitions to a coastal plain. To the south, it is delimited by the Sahel, a belt of semi-arid tropical savanna around the Niger River valley and Sudan Region of Sub-Saharan Africa. The Sahara can be divided into several regions, including the western Sahara, the central Ahaggar Mountains, the Tibesti Mountains, the Aïr Mountains, the Ténéré desert, and the Libyan Desert. Its name is derived from the plural Arabic language word for desert ( ).