Thang Hen Mountain lake, in Trà Lĩnh district, consists of 36 small natural lakes located on the top of a hill at 1,000 metres (3,300 ft) elevation. The lakes have been created in limestone formations and some of them flow out as steams. An unusual feature of the lakes is that their water level fluctuates like the tides of the seas, which phenomenon is attributed to the springs of water emerging from artesian wells in lime stone formations. The lakes flow through the river, which eventually becomes the Ban Gioc waterfall.[26]