Tham Nam is the highlight of the cluster of caves north of Vang Vieng. The cave is about 500m long and a tributary of the Nam Song flows out of its low entrance. As with other caves in the area, there's a zipline and basic restaurant.
In the dry season you can wade into the cave, but when the water is higher you need to take a tube from the friendly woman near the entrance; the tube and headlamp are included in the entrance fee. Dragging yourself through the tunnel on the fixed rope is fun.