Cybertron-Unicron warp gate

The Unicron-Cybertron warp gate is a concealed warp gate linking the lower levels of Cybertron to the nethermost regions of Unicron. It is used by Unicron's minions to pass covertly from Cybertron to Unicron without giving away the Devourer of Worlds' super-secret hidden location. (Psst! He's disguised as one of Cybertron's moons!)

The gate is situated in the middle of a corridor in a seldom-traveled section of Cybertron which was long ago converted into a "fused organic" state. The gate is invisible until you make physical contact with it, and unless you knew it was there, it would be all too easy to stumble through it and find yourself stranded in the bowels of Unicron, surrounded by deadly bot-spiders.

It is strongly implied that the gate is unidirectional (Cybertron-to-Unicron), though circumstantial evidence favors Unicron's minions being able to reverse its function to return to Cybertron.

The fused-organic section of Cybertron where the gate is located is notably atypical, and its transformation is likely a result of slow "leakage" through the gate over a long period of time. Though the gate's exact age is unclear, at least one lifeform (the bot-spider) seems to have migrated through it over the years. The warp gate may date from the original transportation of the Mini-Cons to Cybertron 4 million years ago.

Armada cartoon
While pursuing Thrust, Starscream and several others passed through the Cybertron-Unicron warp gate and had to fight their way up to Unicron's surface to escape.

The Autobots subsequently blocked the warp gate with a large rock, either to prevent its use or merely to prevent people from accidentally wandering through it. When High Wire and the Street Action Team realized their presence was needed to aid Optimus Prime and Galvatron (who had traveled to Unicron by more conventional means), the Autobot Red Alert unblocked the entrance, allowing them and the humans Rad, Carlos, Alexis, Billy and Fred to travel to Unicron's interior.