The central belt of the country is made up of a vast plateau that has an average height of 600 meters, which descends gradually southward towards chotts, an area largely about 350 km from west to east, salty lake basins located partially under the sea level (lowest point is Chott el Gharsa, at -17 metres), the largest of which is the Chott Djerid (7 700 sq km). The south of Tunisia is dominated by the Sahara desert, which alone accounts for about 40% of the area of the country, the western part is occupied by sand dunes of the Grand Erg Oriental. The Tunisia presents 1 150 km of coastline along the Mediterranean Sea, many islands are located along the east coast, including the Kerkenna, Kuriate and the island of Djerba, the largest (514 sq km) of Tunisia. Tourism, thanks to the beauty of the beaches and rich historical and archaeological sites, is an important source of income for the country, from the point of view of tourism, Tunisia is one of the most popular of North Africa. Economy is primarily agricultural, are grown cereals, grapes, tomatoes, olives, vegetables, almonds, citrus fruits and dates, even farming has some importance, breed sheep, goats, cattle, fishing is very important in particular tuna, sardines and sponges. Among the natural resources are especially phosphates, followed by oil. Crafts is developed particularly carpets, material, and the work of jewellery.