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. To get here, come along a scenic but unpaved road to the village of Ban Na Thong. From Ban Na Thong follow the signs towards the cliff and climb a stiff 200m through scrub forest.