Djenné is a UNESCO World Heritage Site located in the Niger River Valley in the country of Mali. It is considered to be among the oldest urbanized centers and the best known archaeological site in sub-Saharan Africa, located about 3 kilometres away from the modern town of Djenné, and is believed to have been actively involved in long distance trade and possibly the domestication of African rice.The site is believed to have exceed 33 hectares (82 acres); however this is yet to be confirmed with extensive survey work. The site is known to have been occupied from 250 B.C. to 900 A.D. The city is believed to have been abandoned and moved where the current city is located due to the spread of Islam and the building of the Great Mosque of Djenné. Previously, it was assumed that advanced trade networks and complex societies did not exist in the region until the arrival of traders from Southwest Asia. However, sites such as Djenné disprove this, as these traditions in West Africa flourished long before. The history of the modern town of Djenné is closely linked with that of Timbuktu. Between the 15th and 17th centuries much of the trans-Saharan trade in goods such as salt, gold and slaves that moved in and out of Timbuktu passed through Djenné. Both towns became centres of Islamic scholarship. Djenné's prosperity depended on this trade and when the Portuguese established trading posts on the African coast, the importance of the trans-Saharan trade and thus of Djenné declined.