Thang Hen lake is located in Quoc Toan commune, Tra Linh district, Cao Bang province. It’s situated on an altitude of thousands of meters above sea level. Thang Hen Lake is a beautiful lake among 36 large and small lakes in the mountainous area of ​​Tra Linh district. It’s one the way from Cao Bang city to the famous of Ban Gioc waterfall.

Thang Hen Lake is a serene mountain lake with beautiful sapphire-blue water. During the dry season, the water level drops making it 36 different lakes. However, the best time to visit is the wet season, when the water level rises and links up all the lakes together to become one big lake of 3 km in length and 1km in width. The lake is surrounded by mountains up to 660m covered in lush forest which is reflected in the water’s surface. Unlike other lakes that become brown in colour during the rainy season, Thang Hen Lake stays blue all year round since it is fed by clear pure water from a cave located above the lake.