The Cybertonium Pit was a location on the planet Cybertron where the substance Cybertonium was mined. The location was encircled by a protective Decepticon force field which prevented prisoners from escaping and forced them to mine the green crystals for their slave masters.
The Cybertonium Pit is made of rock, which is consistent with other instances of the deeper levels of Cybertron being non-metallic, notably the asteroid-origin of Cybertron associated with Primus.
The Transformers cartoon
After the Dinobots accidentally arrived on Cybertron through the Space Bridge, they were attacked by Shockwave's centurions who managed to capture all of the Dinobots except Swoop. Shockwave reprogrammed the Dinobots to be docile and placed them in the Cybertonium Pit to mine the substance for Swoop. When Spike Witwicky, Carly and Swoop were also caught, they were banished to the pit. They were able to "fix" the Dinobots and stage a jailbreak from the pit.