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.
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.
When the bell stopped ringing, the tube was removed and the tomb sealed. You can read more about this practice here.