Nyujo is one of the ultimate ascetic practices of Shingon Esoteric Buddhism.
It is a means by which a priest can save all creatures until the advent of Maitreya beyond the border of life and death. His body will appear later as sokushinbutsu (Buddhist monks or priests who allegedly caused their own deaths in a way that resulted in their being mummified). Nyujo was prohibited by law in the Meiji period. Because it takes many years after Nyujo until the body gets completely mummified as the sokushinbutsu, it is believed that many sokushinbutsu still remain buried underground.
The way of the ascetic practice
Perform Mokujiki practice (fasting) first.
Control the body so as not to rot after death.
Stop eating grains such as rice and wheat, and manage to survive on water and nuts.
Dochu nyujo (contemplation under the ground) is performed next.
Provide a rock chamber under the ground, and enter it.
Completely bury the rock chamber except for a vent made of a bamboo tube.
The priest rings a bell and chants sutra without eating in the rock chamber, but no sound can be heard in time, and it is believed he will appear after a very long time.