Agadez (Air Tamajeq: ⴰⴶⴰⴷⴰⵣ, Agadaz), formerly spelled Agadès, is the fifth largest city in Niger, with a population of 110,497 based on the 2012 census. The capital of Agadez Region, it lies in the Sahara desert, and is also the capital of Aïr, one of the traditional Tuareg–Berber federations. The historic centre of the town has been designated a World Heritage Site by UNESCO. Sultanate of Aïr Agadez was founded before the 14th century, and, by growing around trans-Saharan trade, gradually became the most important city of the Tuareg people, supplanting Assodé. The city still sees the arrival of caravans, bringing salt from Bilma. In 1449 Agadez became a sultanate, but was later conquered by the Songhai Empire in 1515, remaining a part of that empire until 1591. At this point, the city had a population of around 30,000 people By then, the city was a key passage for the medieval caravans trading between the West African cities of Kano (the source of the Hausa language which is the traditional lingua franca between different ethnic groups in the city, especially in the area of trade, religion and administration) and Timbuktu, and the North African oases of Ghat, Ghadames, and Tripoli, on the Mediterranean shore. Internal fighting led to the gradual decline of the sultanate, and by the mid-19th century the town was a shadow of its former self. Some contend that Agadez was the furthermost extent of the Ottoman Empire on the African continent until the 19th century, before being occupied by the French colonial empire, though this claim has not been verified by historians. The city was ruled by the French from 1906. A rebellion by Kaocen Ag Mohammed occurred in 1916, but was defeated by French forces. The French, unable to effectively administer this remote region, ruled semi-indirectly via a restored sultan. Later, Agadez became an important location in the Tuareg Rebellion of the 1990s in central and northern Niger. As a result of the Second Tuareg Rebellion, sporadic violence and the displacement of thousands of people affected the Agadez area from late 2007 into 2009.