It's based off a horror manga. What happened in it was that at the side of a mountain holes with human shapes started to appear, and certain people started to feel "drawn" towards then, claiming "This is my hole! It was made for me!", and they crawl inside.

The thing is, once inside after reaching a certain point, the people can no longer go back, it's too tight, and they are pretty much stuck, and slowly move onwards while the hole starts changing in shape ever so slightly. At the other end of the mountain, after a certain ammount of time, new hole with inhuman shapes started to appear, people claimed they hear strange sounds coming from then, and then…

Well that's horrifying. It also makes way more sense then what I was thinking.
I thought it was a reference to this for some reason.