Cybertron, home planet of the Transformers, is a metallic world. Or, in the Multiverse, it is many metallic worlds. Much like the megaplanet which much of the Multiverse is, most Cybertrons have amalgamated themselves into one gargantuan structure. They have managed to merge together - structurally, at least - quite well, although there are many unusual differences between the different planetary regions. In most cases the Transformer god Primus is credited with the creation of this world, through one means or another.
As one might expect, most regions are either claimed by the Autobots or Decepticons. A small minority might be considered true unaffiliated areas; others might be considered barren wastelands forsaken by every faction. One or two might even be primarily controlled by Minicons.
The underground of Cybertron is elephantine and cavernous. They are also rather well populated - in many instances of the Autobot-Decepticon war, while the Decepticons controlled the surface, the Autobots ruled the underground, where the Decepticons' flight supremacy became something of a liability. There are no generally known maps below the first few hundred levels, and it is believed that parts of them are dynamically changing.
Deep below the surface there are several instances of Vector Sigma, the semi-intelligent supercomputer created by Primus to give the Transformers life. They are, naturally, nearly lost relics, if not outright lost, and their positions take a major quest just to find. Even deeper, somewhere around the core of Cybertron is the Chamber of Primus, the various versions having all amalgamated into a single being.
Cybertron is much larger than many canon sources would have one believe. Unlike Generation One portrayals where it is basically the size of a small Earth city, most iterations of Cybertron were actual planets with millions of inhabitants. This has created an even larger super-Cybertron now that they've merged.
This is a bit of a stub article and will probably be considerably enhanced over time.