Shan Jixiang (C), curator of the Palace Museum, also known as the Forbidden City, and Zheng Xinmiao (R), head of the research institute of the Palace Museum, hold up a treasure box at the main ridge of Yangxin Dian, or the Hall of Mental Cultivation, at the Palace Museum in Beijing on Monday, September 3, 2018. The Palace Museum officially started repairing the Hall of Mental Cultivation on Monday. [Photo: VCG]

Xu Chaoying, a research fellow in the department for ancient architecture at the Palace Museum, shows a treasure box at the main ridge of the Hall of Mental Cultivation, at the Palace Museum in Beijing on Monday, September 3, 2018. The maintenance project of the Forbidden City started on Monday. A total of 13 historic buildings, including the Hall of Mental Cultivation, and their affiliated glass doors and wooden screen walls, will be repaired. The project covers 7,707 square meters, with around 2,540 square meters of construction area. [Photo: Chinanews.com]

The construction site of the Hall of Mental Cultivation maintenance and restoration project is seen in this photo taken at the Palace Museum in Beijing on Monday, September 3, 2018. The project is scheduled to finish in 2020, when the Forbidden City celebrates its 600th anniversary. [Photo: Chinanews.com]

This photo, taken on Monday, September 3, 2018, shows part of the eave of the Hall of Mental Cultivation, a building where emperors of the Qing Dynasty (1644-1911) resided and handled state affairs in the Forbidden City in Beijing. [Photo: VCG]

This photo, taken on Monday, September 3, 2018, shows part of the eave of the Hall of Mental Cultivation, a building where emperors of the Qing Dynasty (1644-1911) resided and handled state affairs in the Forbidden City in Beijing. [Photo: VCG]