there were supposed to be three islands to the east of Bohai Bay where the gods lived. One of them was called Penglai Island where a kind of herbal medicine grew, that was supposed to prolong life. The first emperor of the Qin Dynasty wanted to live fovrever. He sent people to the islands to look for the longevity medicine, but failed.
Emperor Wudi of the Han Dynasty also wanted to live an eternal life. He ordered that a big pool be dug at the back of his palace, complete with three artificial hills to represent Penglai and the other islands. So these legendary hills were built in the eastern capital of Luoyang during the Sui Dynasty and also in the Tang Dynasty capital of Chang'an. Such a traditional style of gardening was followed during the succeeding dynasties and Beihai Park was built after this traditional style.