Yansai Lake (Yansaihu) is a long, narrow reservoir set among low limestone mountains about 8 km north-west of Shanhaiguan, formed by the damming of the Shi River in the 1970s. The water is a deep blue-green colour and the surrounding hills are dotted with watchtowers of a now-flooded section of the inland Great Wall.
The main attraction is the boat trip that runs the length of the lake — a leisurely 90-minute round trip that passes through several narrow rocky gorges, past temple ruins and a couple of watchtower remnants poking above the water. Several lakeside restaurants serve grilled fish and Hebei country cooking. The lake is a quiet escape from the more crowded Great Wall sites and makes a good half-day outing in fine weather.