The Underforge has two exits: One by Jorrvaskr, and one out by Chillfurrow Farm. If you play as a werewolf(And if you can access the Underforge, you play as a werewolf), the secret cavern serves as a safe space to transform away from any prying eyes. This is useful when shifting to Beast Form, but if you've just gotten back from a rampage and can manage to get up to the cave, you'll find that there's no way back in. Until now, that is.

This mod replaces the dummy exit door with an automatic door. Care has been taken to avoid zeroing out any worldspace data(My personal mission with Bethesda modding), and to have the entry door place the player within the bounds of the Underforge, inside of a rather convenient nook.

Recommended werewolf mods are Moonlight Tales Essentials and Extra Werewolf Perks, which, among many other excellent features, contain boosts to jump height that allow the player to actually reach the rear entrance.