Bokura ga Chijou ni Kaeru Toki
ぼくらが地上に帰る時
-Description
On the western side of Yonezawa in Yamagata Prefecture stands the towering Mount Nadera. Something strange happened to me while I was walking through the mountain — I fell into a hole in a grassy field.
Determined to find my way back to the surface, I met a man who claimed he’d been trapped in the cave for five years.
It was published in Manga Shōnen (マンガ少年) on February 1977