On the west side of the Forbidden City is a wide expanse of water called Tai Ye Chi. Tai Ye Chi is divided into three parts: Beihai Lake in the north, Zhonghai Lake in the middle, and Nanhai Lake in the south. Beihai, the best known of the three, was turned into a royal garden as early as 1,000 years ago.
The Designs of the three lakes can be traced to an ancient legend, which says that there were three celestial mountains in the Beihai Sea inhabited by immortals. Rulers took to constructing "celestial mountains" on the water space just outside the royal palace, turing legend into reality.