Why is Wu Tong Blue Mountain in Qiantang a lake?

The reason why the Blue Mountain in Wu Tong, Qiantang is a "lake" on the map is that the Blue Mountain in Wu Tong, Qiantang was not a high mountain, but a low-lying farmland, with an altitude lower than the surrounding area 1 m or more. The update of map data is usually slow. Although residential areas have been built, the map data is still in the original depression.