Undergrid

The Undergrid is a location far beneath Iacon on Cybertron.

IDW Generation 1 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. 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. The Autobots later found that Zeta Prime and the New Senate had kept a secret stockpile of outlawed weapons within the Undergrid.

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.