From the Kircher Society blog, here is the story of the self-mummying suicidal monks:
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.