Timbuktu is a city in Tombouctou Region, Mali. Sankore University and other madrasas are in the city. The city was important for thinking and for religion in the 15th and 16th centuries. It was important in spreading Islam through Africa at that time. There are three great mosques: Djingareyber, Sankore and Sidi Yahya. They are reminders of Timbuktu's golden age. They are always being fixed, but are threatened because the desert is spreading.