‘Sokushinbutsu’, basically a very elaborate method of ritual suicide.
They would seal themselves off in what was basically a sarcophagus and ring a bell daily. If their disciples didn’t hear a bell it meant the monk had passed. The monks believed they were entering a form of suspended animation and expected they would wake thousands or even millions of years in the future.
The Monks Who Spent Years Turning Themselves into Mummies—While Alive
Between 1081 and 1903, at least 17 monks managed to mummify themselves. The number may well be higher, however, as it is likely some mummies were never recovered from the alpine tombs.
These monks undertook such a practice in emulation of a ninth-century monk named Kūkai, known posthumously as Kōbō Daishi, who founded the esoteric Shingon school of Buddhism in 806. In the 11th century a hagiography of Kūkai appeared claiming that, upon his death in 835, the monk did not die at all, but crawled into his tomb and entered nyūjō, a state of meditation so profound that it induces suspended animation. According to this hagiography, Kūkai plans to emerge in approximately 5.67 million years to usher a predetermined number of souls into nirvana.
The process of self-mummification is long and arduous, taking at minimum three years of preparation before death. Central to this preparation is a diet called mokujikigyō, literally “tree-eating training.” This diet can be traced through Shugendō to the Taoist practice of abstention from cultivated grains.At the completion of a thousand-day cycle on this diet, practitioners were considered spiritually ready to enter nyūjō. However, most monks completed two or even three cycles to fully prepare themselves. After the final cycle, the devout would cut out all food, drink a limited amount of salinized water for a hundred days, and otherwise meditate upon the salvation of mankind while waiting to die.