That Xieng Tung is the most important religious monument in Muang Sing. It is believed locally that the stupa contains the Adam's-apple of the Lord Buddha. When the stupa was first constructed is still a mystery. To the left of the stupa is a stone stairway that leads to a sacred fountain and a sacred stone. If you follow the path to the right of the stupa heading downhill you might be able to find the old moat and wall that used to surround the site. The biggest festival of the year, Boun That Xieng Tung, is held here every year during the full moon of the 12th lunar month (usually November) and attracts a large crowd of people from around the province as well as Tai Lue Buddhists from as far top of a hill 5.5 km from the center of town on the road to Luang Namtha.