This Temple is one of the holliest places in Tibet. It was established by the first buddist tibetan king, Songcen Gampo. He had two wives, one from Nepal and one from China. They both were buddist. Chinese wife, when she came to Tibet, brought a holly statue of Budda with her. That was the statue of Jowo Saqyamuni, depicting the crowned 12 year prince Sidhartha (that is the name of Budda). This princess, she was an astrologer. And she said that buddist faith would have a lot of problems in Tibet as there was a giant demon - godess who lied on Tibetan plateau. So the princess made some calculations and said that a temple should be built in Lhasa to neutralize the demon. The temple should be built in the center of the lake, as, according the calculations, that was the place where the heart of the demon was situated. And the water was the blood of the godess. The lake was drained and the Jock-Khang temple was built. Now You can see that statue of Budda here.