Tainai means womb, while jukei literally means "tree form." Fuji's lava enveloped the massive roots and trunks of trees on the side of the mountain, and when the wood had burned away, what was left became the caves we can see today.

When the Fujiko (富士講) sect gained popularity in the Edo Period (1603-1868), its worshippers would frequent the cave before climbing their sacred mountain, stopping to suck water from the round, dripping rocks at the end of the 20-meter (66-ft) ”Mother Tainai" before turning back to go along the 15-meter (49-ft) path to the "Father Tainai" and then emerging "reborn" into the light of day. There are also a few narrow branching paths, but these have not been traveled in recent memory.

The entrance to the cave itself is indeed very womb-like, erupting out of the back of Utsumuro Sengen Shrine (無戸室浅間神社・Utsumuro Sengen Jinja), more commonly known as Funatsu Tainai Jinja, located in the Funatsu area of Fuji Kawaguchiko Town. Crouching down upon entry, the first 6-meter (20-ft) segment even seems to have veins in the walls.

The approach to the Mother Tainai (母乃胎内・Haha no Tainai) is through a tunnel along which you have to crouch with your knees to your chest, while the final chamber is reached by sliding through a small opening on your stomach.

The Father Tainai (父乃胎内・Chichi no Tainai) is located down another branch that can be reached more or less while standing up, after which you proceed to the exit and emerge 20 meters or so southeast of the shrine.

A transformative experience, the Funatsu Tainai Tree Mold is one of 25 sites included in the Mount Fuji World Cultural Heritage Site recognized by UNESCO in 2013.

Address: 6603 Funatsu, Minami Tsuru District, Fuji Kawaguchiko Town, Yamanashi
Hours: 9 a.m.-5 p.m.