Phong Nha is part of unique set of cave systems in the world in that it has a subterranean river flowing through the cave and out and the river is navigable by boat. This allows the visitor a unique experience to sit in a boat and either paddle or cruise through the cave while admiring the experience.
My family and I experienced Phong Nha in 2005. The starting point is the town of Dong Hoi which is access able from Hanoi or Ho Chi Minh. A short drive brings you to Phong Nha which is a pleasant place to walk among the mountains. The cave is located on the river and boats can be rented to paddle you in. The cave is about 5 miles deep but the boat will take you less than a mile and then you get to walk in dry caves looking at the rock formations.
I have experienced several other similar caves, notably the Yellow Dragon Cave at Wulingyuan, China (WHS) and the Puerto Princesa Cave on Palawan, Phillipines (WHS). Phong Nha is by far the best and most natural with the most unique rock formations.
The Yellow Dragon Cave has become very touristy and feels like Disneyland and the Puerto Princesa Cave in the Phillipines are a big disappointment. The trip to Puerto Princesa is long and bumpy together with a rough boat ride across the bay. The karst formation on the cliffs look beautiful but when they motor you in the cave, there was nothing to see. You paddle in and after 30 mins, you turn around and paddle out.