The cave lies roughly 130 kilometres north of Thakhek, in the karst country of northern Khammouane that maps render variously as Phu Hin Bun and Phou Hin Poun. The Nam Hin Bun does not merely pass the mountain; it enters it. The through-passage runs about seven kilometres, and in places it is enormous - up to ninety metres across and a hundred metres from waterline to ceiling. That is a chamber the height of a thirty-storey building, with a river running along its floor and no light in it whatsoever. Guidebooks reaching for a phrase have settled on calling it one of Southeast Asia's geological wonders, and for once the marketing language undersells the physical fact.

Here is the detail that says the most about the place. The 1920 crossing proved a route existed between the village of Konglor and the settlements of the Natane valley on the far side. Knowing it existed did not make people use it. For decades afterward, villagers who needed to travel between the two continued to walk - a punishing six- to seven-hour route over five mountain passes - rather than enter the cave. The passage was a fact; the dark was still the dark. Whatever the mountain was understood to hold, it was not something a proven shortcut could argue anyone out of. The route was only properly exploited much later.

Somewhere in the interior is a pool that glows a hard emerald green. Local people hold it sacred, and the explanation they give is precise rather than vague: the colour is the skin of the god Indra. It is worth sitting with that for a moment. This is not a place where the sacred was retrofitted onto a tourist attraction. The pool was known, named and explained by the people who lived beside the mountain long before anyone thought to sell a boat ticket, and the explanation they arrived at reaches for a figure of enormous cosmological rank to account for a colour in the dark.

Water carved the 7.5-kilometre passage into the Natane valley over millions of years, working the limestone joint by joint until the river had a private corridor through solid rock. What it left behind is a valley that is genuinely remote and genuinely fragile. Its inhabitants build their year around the rains: during the monsoon, some villages simply go self-sufficient, cut off and coping. Rice, tobacco and now tourism are what money comes from. Several ecosystems overlap in the surrounding karst, holding endemic plants and animals, some of them endangered - the price of the same isolation that once made the mountain a barrier.

Today the trip is made in a longtail boat with a guide and a headlamp, and vendors have set up at the entrance in recent years to sell to the people who come. There is a real tension in that, and it should not be smoothed over: the same isolation that makes Kong Lo extraordinary is what visitor numbers slowly erode. But the crossing itself has not been tamed. The engine noise vanishes into a volume of air the light cannot reach, the boat scrapes over shallows where everyone gets out and walks, and the far portal appears as a green coin of daylight long before you arrive at it. Five men rowed this in 1920 with torches that were always going to run out. It is difficult to make that journey small.