Phong Nha Cave is one of the biggest in the world and features an underground river and impressive heights. The inside of the cave is reached from the town of Phong Nga by boat. After a short ride 1km deep, you get to walk for a few hundred meters in one of the biggest grotto.
Although this is a big tourist spot (at least for Vietnamese people, not many western tourists) and the cave is very well lighted, it is indeed an incredible sight.
Reachable from Hué, I went there as a day trip between Hué and Hanoi. I would however advice spending a night and taking time to visit Paradise Cave which I regret not doing.View the set on Flickr »