This green dome is built above a cave that is considered the holiest Muslim site in all of China. The Uyghur people who inhabit Xinjiang (or East Turkestan, as it was known during a brief period of independence during the Chinese civil war) were devout Buddhists before they converted to Islam many centuries ago.
According to legend, the king of Turpan, who was a Buddhist, felt threatened by Islam and sent soldiers to chase down the first three Muslim missionaries that came to the region. They were eventually cornered here and sought refuge in the cave. The soldiers thought they would wait them out, but they were miraculously transformed into doves, and flew out of the cave and over the soldiers heads to freedom.
Today, Chinese Muslims often take pilgrimage here, since it is difficult to obtain permission from the government to travel to Mecca.