Thang Hen Lake

In the limestone mountains of Cao Bằng, 36 lakes lie among the rock. Each has its own shore, yet they join one another through caves under the ground. The water in them rises and falls with the rains. Between the wet season and the dry season, the level can differ by 15 to 20 meters.

This is the Thang Hen lake system, and Thang Hen is the main lake and the largest of them.

The lake lies in Quốc Toản commune, about 30 kilometers from Cao Bằng city. According to the province's water records, it stands 500 to 600 meters above sea level. It sits high on the limestone plateau of eastern Cao Bằng. Among forests and the villages of the Tày and the Nùng, the 36 lakes follow one another like a string of jade.

The Tày and the Nùng are two peoples of northern Vietnam with closely related languages and similar cultures.

The province's official portal explains how the lakes came to be. Over a very long time, water wore away and hollowed out the limestone mountains and left valleys behind. Clay mud settled on the valley floors and kept the water from draining away. So the water gathered into lakes. The province's water records list the seven largest lakes in the system, and Thang Hen is one of the seven.

UNESCO regards this lake system as an important feature of the Non Nước Cao Bằng UNESCO Global Geopark. A UNESCO Global Geopark is an area with landscapes of international geological importance, managed for protection, education and sustainable development. The lakes show how water moves through limestone country. It seeps down through caves under the ground, comes up again, and drains away, following the rhythm of the rock layers and the weather.

What makes Thang Hen special is not only its beauty but also how surprising its water can be. The province's water records say the lake rises and falls twice each day. Around September and October, the water can drain away and fill again in a short time. This is not a legend. It is a real feature of this limestone karst country, because the lake is joined to streams that run underground below it.

The name Thang Hen is often explained, in the Tày language, as "bee's tail," which suggests the shape of the lake seen from above. No careful study has confirmed this, and people in the villages understand the name in several other ways.

On the shore, the green water changes color with the hour and the weather. The cliffs around it reflect in the lake, forest wraps the shore, and farther off lie the villages of the Tày and the Nùng.

Nearby, in the same karst landscape of Quốc Toản and Trùng Khánh, a hole runs through a mountainside and looks like a giant eye. It is known as Mắt Thần Núi. Together, the lakes and the eye tell a story of limestone, of water, and of a span of time too long to picture.

The 36 lakes join through caves under the ground. Their water rises and falls with the rhythm of earth and sky. In the early morning the surface lies flat and still, holding the green forest and the cliffs, while below it the underground streams go on moving by their own law. Nature needed hundreds of millions of years to make all this.