Vietnam: Phong Nha NP – Phong Nha Cave
This is the oldest discovered cave in the national park and it was named after it. The cave is over 44km of passages and 14km of that is a big underground river – Song River. You literally ride into this cave on a boat and cruise in mostly complete darkness for the first 1.5km that is open for tourists. Then you disembark and return to the entrance via several dry passageways, full of massive stalactites and stalagmites and columns. Not the most spectacular perhaps as far as caves go, but the huge underground river is definitely unusual.
