The vast Tham Phu Kham is considered sacred by Lao and is popular largely due to the lagoon in the cave. The beautiful green-blue waters are perfect for a dip after the stiff climb. The main cave chamber contains a Thai bronze reclining Buddha, and from here deeper galleries branch off into the mountain.
Get here via a scenic paved road to the village of Ban Na Thong. From there follow the signs towards the cliff and climb 200m through scrub forest.