The Sub-Tunnels are a series of four worlds in FNaF World, commonly known as the "Glitch Worlds", that the player must go inside in order to take shortcuts to other places of the overworld. Fredbear explains them in the beginning of the game. In the rest of the game, he will warn the player to not go down to more than 3 glitches, as the player will be stuck in the 4th sub-tunnel.