The unmapped, largely unexplored levels immediately below Cybertron's disused sewer system are known as Underworld, home to all sorts of horrible things.
The Underworld was a place of great mystery and danger, dark things lived there and hunted the unwary. In the final days before the war, Crystal City had to deal with dangerous incursions into their underlevels from creatures of the Underworld.
In the old days, after being birthed from the Well of All Sparks, the newly sparked Cybertronian was expected to make a death-defying run through Underworld as a test of courage and worthiness. As this custom died away, Cybertronians turned to another sport full of thrills and spilled oil: gladiatorial combat. This was the first place many disenchanted Cybertronians caught their first glimpse of a fellow named Megatron.
As Cybertron fell into chaos during the civil war, more and more Underworlders began to brave the surface and terrorize the Autobots. Megatron and Shockwave made a deal with them, and the Underworlders allowed the guards of Kaon Prison to place its prisoners underground.