The Phantom Zone was used heavily thousands of years ago to exile criminals and spies, but when General Zod was sentenced, it had been abolished for nearly three hundred years due to the criminals coming out of it at the end of their sentences having gone stark mad... and those who were rehabilitated were dreadfully outdated and had little to offer to Kryptonian society so the practice was seen as harsher than the death penalty.
With Zod, there was too much demand for his head among others in the galactic scene, and since Krypton could not just overturn their death penalty on a whim, they dusted off the Phantom Zone.
Inside the Phantom Zone, is a desert with a glowing sky, giving it the constant impression of twilight. There is a harsh wind and simply breathing is a chore as the air burns your lungs... yet while you feel the pain there is no damage. And while you feel the passage of time, you remain in a state of stasis, never aging.
Other criminals in the Zone remain there, some still serving their original sentences or having been lost to the Zone and unable to leave. Some have made a life in the Zone, forming wretched communities and tribes, while others live in solitude... others still bury themselves in the nether sands, hoping to suffocate... to no avail. Senses are muted in the Zone. What passes for food is bland and without flavor and the constant winds prevent much in the way of meaningful conversation.
There are a few buildings... ruins mostly, from the first few attempts to use this Zone as something other than a prison, but if the winds didn't destroy it, then the residents did out of rage and/or boredom.
In the Zone, other prisoners can maim and hurt you, but the damage is not lasting and never fatal. All non-living things may rot and rust, organics heal within minutes, reverting to the state they entered the Zone in. In Krypton's crueler days, prisoners were starved or worse before being tossed in, so that they might suffer that constant pain for centuries. If someone stays there for far too long he turns into a Phantom.
Since the Fortress of Solitude on Earth-27 is an ancient colonization vessel, it was equipped with a Phantom Zone generator for the sentencing of the colonial prisoners, should the colony get up and running. This is how Zod and his subordinates found their way to Earth, when Clark and Karen were experimenting with their PZ generator as an alternative means of dealing with criminals like Joker, Brainiac, and Vandal Savage.
Trivia and notes
- The Phantom Zone contains Fort Rozz, an ancient Kryptonian detention facility.
- The Phantom Zone generator was stolen by Task Force X.
- Doomsday was imprisoned in the Phantom Zone by Power Girl and was released by the Anti-Monitor.
- Copperhead, Joker and Riddler were sent there by Amanda Waller. Riddler managed to escape.
- Fort Rozz being inside the Phantom Zone is a nod to the Supergirl TV series.
- Prisoners turning into Phantoms is a nod to the Smallville TV series.