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 that 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 base for visiting the cave is Ban Nong Ping, a mixed Lao Loum/Salang village about 2km downstream from the cave entrance. Homestays are available as part of an organised tour with ecotourism operator Green Discovery. With a week or so advance notice, you can organise a trip here, starting from about US$265 for a larger group of six or more to as much as US$550 per person for a couple.