Keng Tao Cave

Water does not always flow where we can see it. In Bắc Sơn, a stream called Mỏ Mắm runs into a mountain and is gone. The place is Chiến Thắng, in Lạng Sơn Province.

The stream begins at Khau Hường Lake, in the village of Hương Cốc. It carries cobbles and boulders of volcanic rock downstream. Then, along one stretch, the stream vanishes. It slips into the limestone mountain and runs underground for more than 400 meters, through Keng Tao Cave, before it comes out again as water on the surface.

That is why the Lạng Sơn Geopark gave this stop on its tourist route number 2 the theme name Underground River. The name is not chosen at random. It describes exactly what is special here: a stream that disappears into the rock and returns, like a riddle of geology that nature has written across the land of Lạng Sơn.

Keng Tao is not a cave of glittering stalactites, or of huge chambers lit with colored lights. It is a cave of science, a geological cave, a place where people come to understand better how nature works. According to the geopark's description, the cave has at least two levels. It grew along a geological fault that runs from northwest to southeast.

That fault created a large zone of tectonic breccia, which can be seen on the ceiling and the walls. These are fragments of rock that were shattered, shifted and rearranged by the movements of the Earth's crust over millions of years.

Standing before the water as it flows between limestone and large fallen blocks, you are looking at the last part of a journey the Mỏ Mắm made in darkness. The water enters from the far end of the cave. It creeps through narrow cracks, passes through the limestone, and appears here. Across those 400 dark meters, the water has been at work. It wears the rock away, carries sediment along, and dissolves calcite. Little by little, it cuts its hidden channel deeper. This has gone on for thousands of years, and it is still going on.

In July 2024, experts from UNESCO were welcomed at Keng Tao Cave with a program during their field visit to assess the Lạng Sơn Geopark. A UNESCO Global Geopark is an area with landscapes of international geological importance, managed for protection, education and sustainable development. The experts' presence here shows that Keng Tao is seen as one of the places that stand for the geological value of the whole region.

Outside the cave, thick vegetation along the stream belongs to the landscape as much as the rock does. Trees and shrubs cling to the water and make a cool, green space all year round. It is not primary forest in the narrow sense. It is the kind of streamside ecosystem that belongs to limestone mountains, where water, rock and plants depend on one another.

Keng Tao has no pretty names for its stalactites and no legend painted on after the fact. Its beauty lies in the plain truth: real water, a real fault, real limestone, and a geological process happening in front of our eyes. Somewhere in the dark inside, the Mỏ Mắm is still flowing, still wearing away the rock a millimeter at a time, as patiently as it has for thousands of years.