Tham Kong Lo is a cave you cross rather than enter: the Nam Hin Bun river runs into a limestone mountain in central Laos and does not surface again for some seven kilometres. Inside, the passage widens to about 90 metres and lifts to nearly 100, a hall of black water and stone cut from marine limestone that was later folded up into the karst of Khammouane Province, now protected as the Phou Hin Poun area. For the villages at either end the tunnel was never a wonder but a road. The overland alternative was a six- or seven-hour walk across five mountain passes, so people carried rice and tobacco through the dark by torchlight instead. Boats only began carrying visitors in 2002.
They leave from Ban Kong Lor: narrow wooden longtails that take three passengers, a boatman standing at the stern, headlamps for light. A few minutes in, everyone climbs out to walk a lit gallery of stalagmites and columns while he hauls the empty boat up a shallow rapid; then the motor picks up again and the beams sweep the water and find no ceiling at all. Close to an hour later the darkness thins and the boat slides out into the Natane valley — rice paddies and tobacco fields walled in by karst, the river pouring from the rock behind you into a pool of deep green. The river still sets the terms. In the wet months it rises and runs fast, and after heavy rain the cave closes; by the end of the dry season it can run so thin that passengers step out and wade while the boat is dragged over gravel. The road in is paved now and Kong Lor anchors the Thakhek Loop, but the crossing has barely changed: a boat, a lamp, and a mountain overhead.