The unmapped, largely unexplored levels below Cybertron's surface are known by multiple names, such as the Underworld and the Undergrid. It's home to all sorts of horrible things.

Fiction

Generation 1 continuity family

Marvel The Transformers comics

It was once traditional for Autobots to undergo a "trial by fire" in the Underworld, but that tradition ended around the time the Ark was launched, 4 million years ago.

The headstrong Autobot Military Academy 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. Underworld!

2005 IDW continuity

Long before the war, the Matrix of Leadership was stolen and hidden away in a vault in the Undergrid by a scientist who refused to continue using it for the purpose of cold construction. The Divided Self There it remained until the time of Zeta Prime's rule, when Orion Pax was badly wounded and cast into the Undergrid by Megatron. There Orion discovered the ancient vault and retrieved the Matrix, which turned him into Optimus Prime. Transformation The Autobots later found that Zeta Prime and the New Senate had kept a secret stockpile of outlawed weapons within the Undergrid. Endgame

Four million years later, on his anniversary of becoming Prime, Optimus returned the remaining half of the Matrix to the vault where he'd found it. Forever Is a Long Time Coming

Aligned continuity family

Aligned novels

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. Exodus