It was declared a World Heritage Site in 1999.Merv first became a significant center under the Achaemenian Empire, and across the millennia which followed was a regional capital for a succession of controlling dynasties. It was from Merv in the 8th century that Abu Muslim proclaimed the start of the Abbasid revolution. At the height of its importance as the eastern capital of the Seljuk Empire in the 11th and 12th centuries, it was a vital center of learning. Here Omar Khayyam worked on his celebrated astronomical tables. Merv was sacked by the Mongols in 1221 in devastating fashion. It remained in occupation during the Mongol period and later, under the 15th-century Timurid leader Shah Rukh, a new city was established here.