Kong Lor Cave is a river cave seven kilometres long in the Khammouane province of central Laos, where the Hinboun river runs through a limestone mountain in the dark, and it is crossed by longtail boat, three passengers and a boatman, from the village at one mouth to the valley at the other, used by the people at either end as a road, since the Natane valley beyond is reachable only through the cave or over five mountain passes. The villagers below the mountain are said to have found the tunnel in the sixteenth century after an unfamiliar duck turned up on the pool at its mouth, and the first outsiders to record the crossing were two French officers taken through in a villager's dugout by bamboo torch in 1890; the boat enters from a pool of green water at the Kong Lor end, the motor droning and a pole taking over wherever the river runs shallow, and the headlamps reach the water and the walls but never the roof, which is gauged by the echo of the engine, up to a hundred metres above. Not far in everyone steps ashore and follows a path strung with electric lights through a forest of stalagmites, columns and stone draperies while the boat is hauled past the shallows, the only stretch of the cave seen properly.
After nearly an hour of black water the passage brightens and the longtail slides into daylight at the Natane mouth, where the river spills from the rock into a pool clear enough to count the stones on its bottom. One pool in the cave glows a bright emerald that the villagers hold sacred, the colour said to be the skin of the god Indra. The valley at the far mouth turns gold in October and November when its rainfed paddies are cut, and the Kong Lor end has guesthouses in the fields under the karst, three hours from Thakhek on the loop road. Kong Lor is a river that goes through a mountain, and the boat is the road.