What is the title to this strange movie description?

A janitor stumbles across a closet (cupboard?) that leads to a cave (tunnel...?) he continues to crawl until the ground becomes fleshy. He finds himself in someone's mind, or something like that. I've never seen the movie personally, but my friend has, and won't stop talking about it. She doesn't know the title, so if you do, PLEASE HELP!!! Thank you.