This mummification was did by Japanese Buddhism, which is called Shingon Buddhism. Sokushinbutsu is a smalle group of the monks in Yamagata, Japan. This is about who is became a buddha by themselves, which is a self-mummufication for hoping save all mankind. But this religious austerities are most difficulty in Buddhism. And in the present, there are about 24 individuals self-mummification were successful.
This is one of the famous Japanese monks, Daijuku Bosatsu Shinnyokai-Shonin, and he was a follower of Shugendo.
The monks would adopt a diet, which is called mokujikigyo (also called "three-eating). This practice takes three years, and the monks eat only tree roots, nuts, berries, etc. The monks lose a lot of weight, and also fats and muscle as well. The reason why they do is remove subcutaneous fat and moisture so make the mummified body easily.
After three years, they do final practice, which is that bury themselves. This practice is the monk digs a hole in the ground by themselves, build a stone hut, set a bamboo cylinder up for breathing, and bury. Everytime the monk rings a bell to tell other monks alive, and finally if they cannot hear the bell, which means the monk has died.
However, the process did not work for every single monk, and some monks cannot wait the long time, and also feel pain to do self-mummification.
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