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Scattered throughout Northern Japan are two dozen mummified Japanese monks known as Sokushinbutsu. Followers of Shugendô, an ancient form of Buddhism, the monks died in the ultimate act of self-denial.
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, where 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.
Not all monks who attempted self-mummification were successful. When the tombs were finally opened, some bodies were found to have rotted. These monks were resealed in their tombs. They were respected for their endurance, but they were not worshiped. Those monks who had succeeded in mummifying themselves were raised to the status of Buddha, put on display, and tended to by their followers. The Japanese government outlawed Sokushunbutsu in the late 19th century, though the practice apparently continued into the 20th.
Source (with photos!): The Self-Mummified Monks of Japan