The most impressive, and yet least visited, cave in Khammuan is the amazing Tham Lot Se Bang Fai. Located in Hin Namno NPA, the cave results from the Se Bang Fai river plunging 6.5km through a limestone mountain, leaving an underground trail of immense caverns, impressive rock formations, rapids and waterfalls that have been seen by only a handful of visitors.
The cave wasn't professionally mapped until 2006, and the Canadian-American who led the expedition concluded that Tham Lot Se Bang Fai is among the largest river caves in the world. Traversing the entire cave involves eight portages and is only possible during the dry season, from January to March. Local wooden canoes can only go as far as the first portage, about 1km into the cave, making inflatable rafts or kayaks the only practical option for traversing the entire length of the cave. The website www.hinnamno.org is a terrific source of information (and legend) on the cave.
The base for visiting the cave is Ban Nong Ping, a mixed Lao Loum/Salang village about 2km downstream from the cave entrance. The village now has electricity, and has built a tourist lodge (80,000K per person), although visits are really only possible outside of the rainy season. To get there, you'll need to make your way along the approximately 40km of very rough road (passable only during the dry season) from Lang Khang to Bualapha, then via an even rougher road to Ban Nong Ping. This is a trip that can only be done with an off-road vehicle; don't even think about attempting it on the city scooter you hired in Tha Khaek.