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Arisada Hōin, 300-yr-old “living mummy” at Kanshūji temple (Fukushima)
To become a living mummy, monks had to undergo a long and grueling three-step process.

Step 1: For 1,000 days, the monks would eat a special diet of nuts and seeds, and engage in rigorous physical training to strip the body of fat.

Step 2: For another 1,000 days, they would eat only bark and roots in gradually diminishing amounts. Toward the end, they would start drinking tea made from the sap of the urushi tree, a poisonous substance normally used to make Japanese lacquer bowls, which caused further loss of bodily fluid. The tea was brewed with water from a sacred spring at Mt. Yudono, which is now known to contain a high level of arsenic. The concoction created a germ-free environment within the body and helped preserve whatever meat was left on the bone.

Step 3: Finally, the monks would retreat to a cramped underground chamber connected to the surface by a tiny bamboo air pipe. There, they would meditate until dying, at which point they were sealed in their tomb. After 1,000 days, they were dug up and cleaned. If the body remained well-preserved, the monk was deemed a living mummy.
Unfortunately, most who attempted self-mummification were unsuccessful, but the few who succeeded achieved Buddha status and were enshrined at temples. As many as two dozen of these living mummies are in the care of temples in northern Honshu.
The Japanese government outlawed the practice of self-mummification in the late 19th century.