Sokushinbutsu were Buddhist monks or priests who allegedly caused their own deaths in a way that resulted in their being mummified. This practice reportedly took place almost exclusively in northern Japan around the Yamagata Prefecture. Between 16 and 24 such mummifications have been discovered.
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.