The Phantom Zone was an eerie twilight dimension used by the people of Krypton as a prison for the most dangerous criminals.
The Phantom Zone is a sub-dimension of absolute nothingness; anything that enters it without shielding loses all physical mass and properties. It was discovered aeons ago by the Kryptonians and was initially used as a method of waste disposal. But after the development of nanite-based recycling technology, Phantom Zone technology was incorporated into the criminal justice system, as a means of housing Krypton's most heinous criminals. The means of sending a criminal into the zone is by being projected by a Phantom Zone Projector and is encased in a small square crystal known as a Phantom Zone Cell.
Other Phantom Zone technology were also used in warfare, especially in the conflict against the Gibborim in which Zone weapons had decimated the Kaeron system. Phantom Zone weapons were later used again by Superman when the Gibborim threatened Earth.
General Dru-Zod was the last criminal to be sent into the Phantom Zone shortly prior to Krypton's destruction; ironically, Zod and the other Phantom Zone criminals were spared from the planet's destruction.
The Phantom Zone Cell housing Zod, which was orbiting the remains of Krypton, was accidentally transported to Earth after being caught in a Warp field by Superman's ship the Argo after rescuing his cousin Kara Zor-El. The Zone Cell was discovered by scientist Mary Storm and was brought to Cadmus for study. After many attempts to unlock the Zone Cell that had shown without any results, Storm fired an experimental neutron beam as a last resort but only to turn the Cell into a focal point and creating multiple pre-singularity formations, causing an inevitable scenario of either engulfing Earth into the Phantom Zone or unleashing its criminals. Zod was also freed from his prison after sending a radio signal after it was bombarded by the neutron beam and was detected by Superman. The singularity formations were spread across the United States and creating breaches into the Phantom Zone, allowing its criminals to be freed. The Justice League, X-Men, and X-Factor were dispatched in neutralizing the singularities and the criminals. Only one singularity was obtained by Project Cadmus in an effort to harness its power, but only to inadvertently increase it and creating a Meta-Stability, a very narrow energy-level in which it was increasing in energy and capable of swallowing Earth. At Superman's instructions, the singularity was fed with enough energy from the Cadmus facility it was located to disrupt its Meta-Stability and thereby collapsing on itself.