Tham Nok Ann is a soaring, well-lit cavern through which a river passes beneath awesome rock formations. It's dripping, creepy and very atmospheric, with a set of stairs leading up to an adjacent cave complex that once housed a Vietnamese military hospital. You could previously take a boat trip deeper into the cave, like a mini Tham Kong Lor experience with some huge jellyfish-like rock formations visible, but in recent years it has only been open for viewing on foot due to limited visitor numbers.
Look for a signpost on the main road about 5km before Vieng Xai and follow the small track around to the right until it dead-ends at an entrance booth and small suspension bridge.