This is absolutely wild.
Scattered throughout northern Japan are over two dozen mummified Japanese monks known as sokushinbutsu. Followers of shugendō, an ancient form of Buddhism, the monks died in the ultimate act of self-denial.
Source: Sokushinbutsu of Dainichibou Temple | Atlas Obscura
For three years, the priests would eat a special diet consisting only of nuts and seeds, while taking part in a regimen of rigorous physical activity that stripped them of their body fat. They then ate only bark and roots for another three years and began drinking a poisonous tea made from the sap of the urushi tree, normally used to lacquer bowls. This caused vomiting and a rapid loss of bodily fluids, and—most importantly—it killed off any maggots that might cause the body to decay after death. Finally, a self-mummifying monk would lock himself in a stone tomb barely larger than his body, wherein he would not move from the lotus position. His only connection to the outside world was an air tube and a bell. Each day, he rang a bell to let those outside know that he was still alive. When the bell stopped ringing, the tube was removed and the tomb sealed.