Sokushinbutsu were Buddhist monks or priests who caused their own deaths in a way that resulted in their mummification. This practice reportedly took place almost exclusively in northern Japan around the Yamagata Prefecture. It is believed that many hundreds of monks tried, but only between 16 and 24 such mummifications have been discovered to date. (Via) This practice was folklore until Buddhist monk mummies were discovered in July of 2010.
Sokushinbutsu was tried by hundreds of monks, as it was the celestial trifecta of religious experiences in Japan. Only about 24 monks were successful. The experience of committing suicide by the monks began in an elaborate process of 1,000 days (a little less than three years) of eating only nuts and seeds to eliminate all body fat. Over-zealous exercises were employed.
For another 1,000 days, the monks only ate bark and roots and began drinking a poisonous tea made from the sap of the Urushi tree, normally used to lacquer (eliminate) their bowls. In other words, there was profuse diarrhea occurring among the participating monks. Though the vomiting and diarrhea was important to later mummify themselves, the ideal outcome of drinking the sap of Urushi trees was to make their body poisonous and inedible to maggots. In turn, their body would not deteriorate by natural processes.
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 air tube was removed.
Another 1,000 days ensued. Then, some unfortunate monk was summoned to open the tomb and see if his body was mummified. If all was well, and mummification had been achieved, the monk was declared a Buddha and the body was put into the temple for viewing. Despite the lordship bestowed to the corpse, it was just a decomposed body and nothing more. (Via)
Images from Pink Tentacle.