Entry to the cave area costs 10,000 kip and starts with a walk across a suspension bridge which spans a small river.
Heading left and up the hill you reach a large cave entrance which is the beginning of an old Vietnamese military hospital complex. Massive caverns with interconnecting passageways and staircases give a glimpse of the scale of the hospital where Vietcong soldiers came for treatment.
After exiting the cave, it's possible to get a fantastic aerial view of Nok Ann Cave and the swirling stream below. At this point a decision can be made as to whether you want to leave the cave area entirely or go on a cruise down the stream in a small plastic boat. Small boats capable of carrying two Lao people cost 25,000 kip for the 400 metre journey down the cave river or 40,000 kip for the larger boat capable of carrying two Westerners.
Although some might baulk at the cost of the boat journey, the internals of the cave are stunning and are well-lit so that you can see its unique geological features. The return journey only takes about 15 minutes and is certainly worth a look.
Nok Ann Cave is six kilometres from the Vieng Xai bus station in the direction of Sam Neua and is easily walkable. The turn off is well-signposted and from there you will need to look for the white spray-painted arrows. If you find yourself surrounded by men with rifles, you've gone too far and inadvertently entered the military base.
This is a fantastic attraction.
The Travelfish newsletter is sent out every Monday and is jammed full of free advice for travel in Southeast Asia. You can see past issues here.