During the Koryo dynasty, the thriving Buddhist and aristocratic culture saw its crowning glory of artistic achievement in celadon ware with its delicate jade-green color. The period also saw the mushrooming of Buddhist temples and monasteries, including the Nirvana Hall of Pongjungsa Temple, which is thought to be the oldest surviving wooden structure in Korea. Reflective of provincial culture, the superb Hahoe and Pyongsan wooden masks were originally made to be worn for the Puylsin-Gut shaman ritual, which was held on the 15th day of every first lunar month. During the Choson dynasty, Korean science and technology as well as arts developed its own unique identity by fully integrating Chinese counterparts into Korean traditions. During the reign of King Sejong, the automatically-striking water clock, termed the chagyongnu, was created and An Kyon studied the artistic styles of many notable Chinese and Korean painters to create his own original expression in landscapes like his masterpiece "Dream Journey to the Peach Blossom Land."