Tham Lot Kong Lo, in Khammuan Province, is a 4 km long cavernous tunnel of limestone through which flows the beautiful turquoise waters of the Nam Hin Bun. The tunnel is incredibly wide in places, up to 100m wide and almost as high. The scenery is as stunning inside the tunnel, where at times it feels like being inside a Gothic cathedral, as outside, where the river makes its way through spectacular karst scenery. It is possible to hire a motorised boat to take you through the tunnel and back, which takes about 2 hours. Make sure you bring a torch and rubber sandals as you may have to get off the boat where the water is too shallow and the gravel is sharp. The best way to get here is by hired vehicle from Tha Khaek. You will have to stay overnight near the cave, the best place to stay is at Sala Hin Boun, where beautiful and comfortable Lao-style wooden bungalows are available for around US$20. The scenery all around is stunning and the region sees very few visitors, making it a worthwhile off the beaten track destination.