Cybertron's lower levels are generally uninhabited. The levels immediately below Cybertron's disused sewer system are known as Underworld, home to all sorts of horrible things.
Marvel Comics UK
The headstrong Autobot cadet Flattop convinced Subsea and Tailgate to join him for a "trial by fire" through Underworld. They were attacked by a psychotic gang of heavily-modified Transformers called the Mutants that made their home there. Flattop and Subsea were killed, but Tailgate destroyed the Mutants and escaped, having succeeded in his trial by fire after all.
The Demons passed through the Underworld layer before reaching the surface of Cybertron.