The Phantom Zone is a prison-like parallel dimension appearing in American comic books published by DC Comics. It is mainly associated with stories featuring Superman. It first appeared in Adventure Comics #283 (April 1961), and was created by Robert Bernstein and George Papp. It was frequently used in the Superman comics before the continuity was rebooted in the 1980s, after Crisis on Infinite Earths, and has appeared occasionally since.
The Phantom Zone was a "pocket universe" discovered by Jor-El that existed outside the space-time continuum; it was used on the planet Krypton as a humane method of imprisoning criminals. Kryptonians had abolished the death penalty in the long distant past. In more recent history, criminals were punished by being sealed into capsules and rocketed into orbit in suspended animation with crystals attached to their foreheads to slowly erase their criminal tendencies; Klax-Ar was one criminal who received this punishment but escaped. Gra-Mo was the last to suffer the punishment, for it was then abolished in favor of the Zone.
The inmates of the Phantom Zone reside in a ghost-like state of existence from which they can observe, but cannot interact with, the regular universe. Inmates do not age or require sustenance in the Phantom Zone; furthermore, they are telepathic and mutually insubstantial. As such, they were able to survive the destruction of Krypton and focus their attention on Earth, as most of the surviving Kryptonians now reside there. Most have a particular grudge against Superman because his father created the method of their damnation, and was often the prosecutor at their trials. When they manage to escape, they usually engage in random destruction, particularly easy for them since, on Earth, each acquires the same powers as Superman. Nevertheless, Superman periodically released Phantom Zone prisoners whose original sentences had been completed, and most of these went to live in the bottle city of Kandor.