It's just Mario's tunnel of a doom. This is a joke Easter egg.
You can access it inside the Pirate Caverns after The Face gives you the key, simply hold the Down key to get inside. You will need to survive against the yeast of thoughts and minds and the Human. (Frisk from underpants)
- This was made when I thought about Mario's tunnel of a doom
- To learn about the yeast of thoughts and minds, type in "glob" and if you see a long ass name, click it
- Yandere Mickey will help you out on this night.
- Distorted Mouse will probably sue me for mentionimg his fan made suit.