Merv

Merv was a great oasis city on the Silk Road, near modern Mary in Turkmenistan, and is now a UNESCO World Heritage Site. It was the centre of the Achaemenid province of Margiana, a long-standing centre of Buddhist learning, and from the 5th to the 11th century the seat of a Church of the East metropolitan. After the Arab conquest of 651 it became capital of Umayyad Khurasan; the Abbasid cause was proclaimed there in 748, and the caliph al-Ma'mun resided there from 813 to 818. As a Seljuk capital it may have been the world's largest city around 1150, and Yaqut al-Hamawi counted ten great libraries there before the Mongols devastated it in 1221.