Within Phong Nha National Park lie the world's largest caves. Son Doong cave, the world's largest cave that was recently discovered, will take you thousands of dollars and two years to be able to explore due to a long waiting list. However, a few miles away the Hang En cave (Swallow Cave) is available for exploration and even overnight camping. A day of trekking down through the leech-infested jungle and through many valley streams will bring you to the large mouth opening of this beautiful cave. During the rainy season the river within can be up to 300 feet, but during the dry season the inside is sandy and dry. Within the cave lies a lagoon and two amazingly large entrances. It will feel like you have entered the pages of National Geographic.
The treks are organized within the city of Phong Nha and due to conservation issues must be done with an authorized tour company. Bring waterproof clothing and long pants and shirts due to leeches.