User blog comment:Mr.Peco/Jaggi Nest/@comment-4820236-20120707042813

Well, I can explain this fairly easily.

D. Island Glitch Explaination
See, if you do the D. Island Glitch, you can get into the Area 5 Cave. Your hunter can walk around just fine in here. This means that these tunnels are In-Game background and floor. However, the entrance is designed at a slightly lower hight than the Hunter Sprite, in order to keep hunters from going in. If you look from an ingame perspective, it is just a large undergraund cavern, with openings in assorted places.