Thang Hen Lake is in Quoc Toan commune, Tra Linh district. It is in the north of Cao Bang, near the route Cao Bang - Tra Linh.
Situated among limestone mountains, the lake has a basin position. It is about 500m length, 100m width with lowest water level of 10m. In rainy season, the river very blue water overwhelms mountain bottoms. In dry season, water is exhausted except for Thang Hen Lake of which water never dries. The upstream of this lake is Thang Hen cave where water is blue in all four seasons creating potentials for economic development but also for tourism.