Roman town; and Djémila and Timgad, both Roman ruins; M'Zab Valley, a limestone valley containing a large urbanized oasis; and the Casbah of Algiers, an important citadel. The only natural World Heritage Site is the Tassili n'Ajjer, a mountain range. Transport The Algerian road network is the densest in Africa; its length is estimated at of highways, with more than 3,756 structures and a paving rate of 85%. This network will be complemented by the East-West Highway, a major infrastructure project currently under construction. It is a 3-way, highway, linking Annaba in the extreme east to the Tlemcen in the far west. Algeria is also crossed by the Trans-Sahara Highway, which is now completely paved. This road is supported by the Algerian government to increase trade between the six countries crossed: Algeria, Mali, Niger, Nigeria, Chad, and Tunisia. Demographics Algeria has a population of an estimated 44 million, of which the majority, 75% to 85% are ethnically Arab. At the outset of the 20th century, its population was approximately four million. About 90% of Algerians live in the northern, coastal area; the inhabitants of the Sahara desert are mainly concentrated in oases, although some 1.5 million remain nomadic or partly nomadic. 28.1% of Algerians are under the age of 15. Between 90,000 and 165,000 Sahrawis from Western Sahara live in the Sahrawi refugee camps, in the western Algerian Sahara desert. There are also more than 4,000 Palestinian refugees, who are well integrated and have not asked for assistance from the United Nations High Commissioner for Refugees (UNHCR). In 2009, 35,000 Chinese migrant workers lived in Algeria. The largest concentration of Algerian migrants outside Algeria is in France, which has reportedly over 1.7 million Algerians of up to the second generation. Ethnic groups Arabs and indigenous Berbers as well as Phoenicians, Romans, Vandals, Byzantine Greeks, Turks, various Sub-Saharan Africans, and French have contributed to the history and culture of Algeria. Descendants of Andalusian refugees are also present in the population of Algiers and other cities. Moreover, Spanish was spoken by these Aragonese and Castillian Morisco descendants deep into the 18th century, and even Catalan was spoken at the same time by Catalan Morisco descendants in the small town of Grish El-Oued. The majority of the population of Algeria are Arabs (75% to 85% of the population). Berbers who make up 15% to 20% of the population are divided into many groups