Like others who became sokushinbutsu, Shinnyokai chose the path of an issei gyonin, or one who dedicates his life to ascetic practices. This mainly involved two types of austerities: mountain confinement, called sanro, and tree-eating austerities, mokujiki-gyo.

Ascetics would spend 1,000-day intervals in Mount Yudono’s Senninzawa (Swamp of Immortals), performing cold-water ablutions and visiting the Yudono Shrine as part of their rigorous training regimen. Many would not survive the freezing winters, when the area would be covered with heavy snow. Those who perished were honored with stone tablets commemorating their challenge.

To begin preparations for self-mummification, the ascetics would adopt a strict diet requiring them to abstain from eating the “five cereals” of rice, barley, wheat, soybeans and adzuki beans (this combination could vary), followed by another set of five plants, typically corn, Japanese millet, foxtail millet, buckwheat and potatoes. Practitioners instead would forage in the mountains for nuts, roots and seeds – a process Endo said was aimed at reducing body fat and muscle.

To inhibit the growth of bacteria, Endo said Shinnyokai is believed to have drunk the toxic sap of urushi, the Chinese lacquer tree related to poison sumac. And to prevent decomposition, he consumed nothing but salt water for over 40 days, emptying the body of any food.

Finally, the ascetic would perform dochu nyujo, or entering deep meditation in the ground.

“A pit 3 meters deep and around 4½ tatami mats wide was dug, and its walls were ensconced with stone,” Endo said. “A wooden coffin would be assembled for the ascetic to enter, which would then be lowered into the chamber. Empty space would be filled with charcoal to remove humidity.”

Once the pit was secured shut, two bamboo tubes would be inserted to funnel down drinking water and act as air vents. Bells would be attached on both ends of one of the tubes, a device used by the ascetic to signal that he was still alive. Once the ringing stopped for good, Endo said, the bamboo tubes would be pulled out to seal the pit. For the next three years and three months, the corpse would be left in the underground cell. On the final day, the body would be unearthed — an instant Buddha if all had gone well.

Of course, success was not guaranteed, and many corpses decomposed during the process. There are also said to be numerous burial mounds of sokushinbutsu yet to be excavated after the practice was banned during the Meiji Era.

Endo said Dainichibo used to enshrine three sokushinbutsu but two were lost to fire when the original temple burned down in 1875, seven years after the Meiji Restoration. Now Shinnyokai sits alone in a glass case, except when he is taken out for a redressing ceremony held every six years.

Sokushinbutsu offer a trove of interesting backstories. Honmyokai entered the ground in 1683 to become the oldest mummy in the Shonai region. He was believed to be a samurai who decided to devote himself to a life of an ascetic after praying at the Swamp of Immortals for the recovery of his master, Lord Sakai, the daimyo of Tsuruoka, who was suffering from an illness.

Komyokai, who is said to have been an excellent hunter with perfect aim before entering the priesthood, was discovered in 1978, 124 years after his death. The excavation that took place in the town of Shirataka in Yamagata Prefecture stirred people’s imagination and was widely publicized at the time.

But in terms of the number of colorful episodes and meritorious deeds recorded, no one surpasses Tetsumonkai, whom Hijikata calls the “superstar” of all sokushinbutsu. Born Sunada Tetsu in 1759, he was a river worker who dug wells and floated lumber, and was known for his stormy temperament. According to one account, he killed two samurai after finding them intoxicated on duty and confronting them for negligence. Another account describes him killing a samurai during a fight over a favorite prostitute.

Churenji, the temple where Tetsumonkai is enshrined, gives a somewhat milder account, saying he was angered by the arrogance of an official in charge of river construction and pierced the man’s leg with a fire hook and dragged him around. In any case, Tetsu fled to escape his pursuers and joined the seminary at Churenji in his 20s, where he was given his priestly name.

Records indicate Tetsumonkai was a widely traveled and respected holy man with numerous legends to his name. Once when he was visiting Edo, he witnessed the outbreak of an eye disease that caused great suffering. He proceeded to gorge his left eye out and offered it to the Sumida River in prayer for a cure. Later research found that his left eye is indeed missing, going some way to authenticating the story.

Komei Sato, the head priest of Churenji, has been working to unearth traces of Tetsumonkai through stone monuments commemorating his deeds. Nearly 200 of them have been discovered so far, he said. “There are probably more out there.”

Tetsumonkai’s missionary work centered on the Shonai region, but the monuments show it extended from the Kanto region up through Hokkaido. He is remembered for gathering 10,000 volunteer workers to build a new road through a mountain connecting Kamo Port to Tsuruoka, facilitating trade.

Sato said there are festivals that continue to this day based on Tetsumonkai’s teachings. “He left an enduring impact on many people of that time,” he said.

However, perhaps the most compelling of his legends may be another one involving self-mutilation. At one point, Tetsumonkai is said to have been visited by a prostitute, possibly the same one he fought the samurai for. The woman tried to convince Tetsumonkai to come back to the city with her, but he refused. To prove his resolution and dedication to a life of austerity, he disappeared and shortly returned with a small package for her. Inside were his bloody testicles. He had sliced them off.

The object is said to have made its way around prostitutes of the local pleasure quarters as a good luck charm, and was eventually sent to Nangakuji Temple in Tsuruoka, where it was preserved as a relic. Adding weight to the legend, the genitals are missing from Tetsumonkai’s mummified body.

I visited Nangakuji to see Tetsuryukai, one of the newest surviving sokushinbutsu, who is believed to have been mummified at around 1881. Considered Tetsumonkai’s disciple, Tetsuryukai was preparing for self-mummification when the practice was outlawed. He and his followers secretly went ahead with the process, but it wasn’t until 1923 that his existence could be publicized.

One of the visitors asked a question that had also been on my mind: Was the temple really in possession of Tetsumonkai’s testicles? “Yes,” said the head priest’s elderly wife, who was giving us a brief tour of the room where Tetsuryukai is enshrined along with other artifacts. “But they’re not for public viewing.”

Tetsumonkai’s blood group is B, which was also the blood group of the scrotum found in Nangakuji, according to past scientific research. Academics at the time concluded that it was highly likely that the dried testicles belonged to a man who endured extreme physical abuse in the name of ascetic training before being entombed at the age of 71.

It’s easy to dismiss the sokushinbutsu phenomenon as an obscure ritual that died out as the nation marched toward modernization in the late 19th century. However, the mummies provide a fascinating window into the social landscape of pre-modern Japan through their stories of passion, hardships, sacrifice and intense religious fervor culminating in the attainment of Buddha-hood in the flesh.

And they are, in a sense, still very much alive, treated as living, breathing saints by generations of priests and worshippers who have kept the tradition relevant.

To those of us who visit the quiet mummies today, they offer a respite from the seemingly unending trivialities of life.

“They make us think of life and death, and what, if anything, it all means,” Hijikata said. “I’ve come to believe that’s what they’re here for.”