Dark Cave

Some places send people out with mud in their hair, their clothes soaked through, and a smile that will not fade. The Chày River and Hang Tối, the Dark Cave, make one of those places.

The Chày River and the cave lie inside Phong Nha Kẻ Bàng National Park, on a route run by the park's own Tourism Center. That center is a public service unit under the park's management board. The river comes first.

Phong Nha Conservation records the Chày as the upper course of the Son River. It rises from streams in the national park and runs for about eight kilometers. The water is jade green. On both banks, especially upstream, stand the karst towers that mark Phong Nha Kẻ Bàng: tall masses of limestone rising straight up with their feet in the water. This is the landscape that people often see in photographs when they look up this part of Vietnam.

On the far bank is the mouth of the cave, and it is completely dark. A visitor stands before it, and then steps inside.

Inside, the first thing a visitor notices is that there are no electric lights. The Tourism Center describes the cave as having no artificial lighting at all. That is one of the big differences between this cave and Phong Nha Cave or Thiên Đường Cave. Everything in the cave depends on the headlamp that each visitor brings. The cave is seen the way it really exists, in its own natural darkness.

The Tourism Center also records that Hang Tối was discovered in 1990. It runs for nearly 6,000 meters and rises about 80 meters high. It is a cave that has the features of both a water cave and a dry cave. Phong Nha Conservation explains the geology of the cave. A fault line running from the northwest to the southeast became active again in the Cenozoic era, and it helped to form this cave and several others in the area.

The most special thing about Hang Tối is its mud lake.

The same source explains how it forms. In the rainy season, the water level in the cave rises. As it rises, the water carries soil and sediment from the limestone and the forest outside into the cave. When the water falls back, a layer of fresh mud is left behind, and that layer becomes the mud lake. To bathe in mud inside a dark cave, by the light of a headlamp, is a very rare experience. Many visitors remember this longer than anything else they find along the route.

Hang Tối is a route for people who like to be active and enjoy strong thrills, but still want professional guides. It is not a multi-day expedition like Sơn Đoòng or Hang Én. It is not a leisurely sightseeing walk either. What it offers is a stretch of jade green water, a cave in complete darkness, and a strange lake of mud.

This is the place where the waters of the Chày River connect with the vast system of underground rivers in Phong Nha Kẻ Bàng National Park, a world natural heritage site. Jade green water flows out of the streams in the forest and curls around the upright karst towers. Then it disappears into the earth, somewhere deep inside.