Thang Hen Lake is one of the most beautiful lakes among the 36 large and small lakes in the mountainous region of Tra Linh district. At the head of the lake is a large cave, from which water flows out day and night. One particularly fascinating thing that the locals say is that every 10 years or so, suddenly the water in Thang Hen Lake nearly dries up, and just after a few days, the water rises again. Another impressive thing is that during the flood season, Thang Hen Lake still retains its characteristic emerald green color, while other lakes in the region turn a bright red. The first time I came to Cao Bang during the flood season, I was very shocked when Mount Thung and Lenin Stream were muddy, but Thang Hen Lake was crystal clear.