Phu Hin Bun NPA is a huge (1580 sq km) wilderness area of turquoise streams, monsoon forests and striking karst topography across central Khammuan. It was made a protected area in 1993 and it's no overstatement to say this is some of the most breathtaking country in the region.
Passing through on foot or by boat, it's hard not to feel awestruck by the very scale of the limestone cliffs that rise almost vertically for hundreds of metres into the sky. Although much of the NPA is inaccessible by road, local people have reduced the numbers of key forest-dependent species through hunting and logging. Despite this, the area remains home to the endangered (red-shanked) douc langur, François' langur and several other primate species, as well as elephants, tigers and a variety of rare species of deer.
A trip out to Tham Kong Lor will give you a taste of what the NPA has to offer, but there are two more immersive ways to go deeper into this area of almost mythical gothic peaks and snaking streams.
Khammuan Province runs five different community-based treks of varying lengths. From Tha Khaek, the popular two-day trip (1,700,000K for one person, 950,000K each for two, 650,000K for six or more) into the Phu Hin Bun NPA is especially good. The route includes plenty of karst scenery, a walk through Tham Pa Chan and overnight accommodation in an ethnic village. Bookings can be made through the Tourist Information Centre in Tha Khaek.
Green Discovery offers similar treks, including a very tempting three-day kayaking and cycling trip between spectacularly sheer cliffs, as the Nam Hin Bun (Hin Bun River) follows a large anticlockwise arc towards the Mekong.