It is said that a “Sokushinbutsu” is a monk who died through a special, natural process, which we call Live Mummification. This process reportedly only takes place in the city of Yamagata in Northern Japan . 16-24 such mummies have been found.
Monks turning themselves into mummies would follow a strict diet of only nuts and seeds to reduce body fat. Three years later, they would only eat barks and roots of trees and drink poisonous saps from certain trees (in special bowls coated with paint). This continues for another 3 years. This dietary regime makes them vomit often, sharply reducing the percentage body fluid and also preventing the growth of maggots; hence preserving their bodies after they die.
Just before death, they fit themselves in a tight-fitting hole that shapes their bodies into the shape of a lotus; the only contact with the outside world is a bell and an air tube. The dying monk rings the bell daily to let others know he is still alive. The day the bell stops ringing, others would remove the air tube, seal the hole, thus entombing the monk.