The Underground Labyrinth (a.k.a. CELL, TRAIN, or 8-Bit Dungeon) is a fairly large maze, home to Chasers, spikes, pits, the Hiding Man, and lots of dead ends. It connects the Red Cave to the Circus. Its walls and floors are littered with dead bodies and blood stains.
It becomes accessible after acquiring all three Keys and triggering the storyline event at the Windmill.
This area features "retro" graphics and music, evoking an NES or GBC aesthetic earning this area the alternate name "8-Bit Dungeon". (Note, however, that this area is not a dungeon.) Rank calls this place an "underground labyrinth" during the NPC Quest, and Mitra calls this place "that dark labyrinth". In the game's code, this area has two names, "CELL" and "TRAIN".