The underground is the section of the finale that comes directly after the Level 2 part of the Final Chapter (?) of T.T.F.N. 2. It precedes Luno's Apprehension directly. It begins with Solvice landing in a hallway with 4 buttons repeating. He must then figure the pattern of the buttons correctly without getting a game over.
This leads into an area with two spike puzzles that Solvice must avoid. He encounters the red chest from Level 1, Doran, who gives him the skills Pistol Fire and Slash, meets a copy of himself, and finds a discarded bandage on the ground and fights Rutger's ultimate form.
It culminates in a dungeon in which Solvice climbs the stairs and has a choice to either kill or apprehend Luno. Apphrehending Luno leads to the first bad ending, and killing Luno results in the game resetting into the glitch areas.