This formidable cave network, 40km north-west of Oudomxay, extends for 16.5km. It is known as Tham Chia, the "bat cave", from which local villagers use the guano as fertiliser. Visitors can explore passageways and tunnels which reach widths of 20-30m and heights of 50m, embellished with impressive stalactites and stalagmites. The River Nam Khang runs through the passages of this cave system, in which only the (southern) dry section can be explored without any special equipment.