The lost ancient culture of China.

Mathematics: Ten classic books on arithmetic, some of which are lost and some of which are incomplete, such as Zu Chongzhi's composition. I don't know if there is any existence beyond the current mathematical theory system.

Medicine: Hua Tuo's Qingnangshu. It is also an ancient medical book close to modern medicine.

Because ancient Confucian scholars hated science, many ancient technologies and secretaries were lost. Seismograph, wooden ox and flowing horse, etc.

Emperor Wu of the Han Dynasty was so exclusive about Confucianism that he really couldn't tell whether it was good or bad for later generations, and many ancient changes were suppressed. It is not necessarily wrong for the first emperor to pit Confucianism.