The practice of the underground palace was not handed down from Buddhism, but combined with the tradition of "being buried in peace" in China's folk funeral. But usually large monasteries have places for monks to go after their death. There are two forms: one is cremation, which is the most widely used and common way, including Sakyamuni; The second is to install a jar (this is the monk who foresees that his practice may achieve positive results and deliberately leaves his body as a tool to educate the world. So when he died, he told his disciples to put his body in a big jar with a hole in the bottom and surround it with charcoal for three years before opening it. If the corpse doesn't rot, it means that he has achieved a positive result, and the disciple will paint it with gold and save it, which is called "golden body". The most typical examples are Hui Neng, the sixth ancestor of Guangdong Province, and Jin Qiaojue, the incarnation of Tibetan Bodhisattva in Jiuhuashan, Anhui Province. If it has rotted, it will be lit and burned from the bottom of the urn.
Therefore, if the cremated ashes belong to the eminent monk Dade, they will be stored in a separate tower called "stupa" (such as Tallinn in Shaolin Temple); In the case of monks and laymen, the stupa is jointly built and stored, which is called "Haihui Pagoda". The underground palace, which you mentioned above as a cemetery, is a comprehensive product of folk funeral and burial combined with Haihui Tower. The ashes are placed here and blessed by the Buddha.