The Ahon Roo Ferro cave system sits at the base of Mount Otokoyama in the Hokkaido region, Japan. It was given the name "Entrance to Hell" by the local Ainu people, after the mysterious death of a man who walked in.
A reporter who found the cave preferred to listen to the instructions rather than go inside. The entrance is fenced, but brave and determined adventurers can break in by crawling under the barrier. The only way the cave could lead to it was down - and from there the way out is unknown.