The Phantom Zone was discovered by Jor-El and used on the planet Krypton as a method of imprisoning criminals. Previously, 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 managed to escape. Gra-Mo was the last to suffer the punishment, for it was then abolished in favour of the Phantom Zone, which was considered more humane and cost-efficient but equally effective. The Phantom Zone criminals first appeared in Adventure Comics (Superboy stories), and soon began appearing in Superman stories. The inmates of the Phantom Zone reside in a featureless 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 are 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. When they manage to escape, they usually engage in random destruction, particularly easy to them since, on Earth, each has the same powers of Superman. Nevertheless, Superman periodically released Phantom Zone prisoners whose original sentences had been completed, and these fortunately tended to be relatively repentant criminals.
Throughout the Silver Age of Comics, Superman meets many residents of the Phantom Zone:
Inhabitants of the fictional planet Krypton appear to resemble humans, but their biology is much more complex. According to the DC Comics canon, the cellular structure of Kryptonians allows for solar energy to be absorbed at extremely high levels when exposed to a yellow star like Earth's Sun, giving them vast superhuman powers (such as flight, strength, x-ray vision, super speed, invulnerability, heat vision and superhearing) when on planets such as Earth.
Visually, Kryptonians are identical to humans; this allows Kal-El to lead a double life as Clark Kent. On their native planet, Kryptonians were not known to possess any superpowers as Krypton revolved around a red star. However, when in the vicinity of a yellow star, such as the Sun, Kryptonians utilize solar energy on the cellular level to achieve abilities unattainable by ordinary human beings. Among other abilities, super-powered Kryptonians possess the following at the peak of their power: flight, superhuman strength and speed, invulnerability, x-ray vision, telescopic/microscopic vision, heat vision, super breath, freeze breath, and super hearing. Kryptonians also have bioelectric fields that surround their bodies and protect them from harm. Grant Morrison has portrayed this field as telekinetically controllable . Such may or may not be the method by which Kryptonians fly. On occasion they have been depicted with "tactile telekinesis": this includes Conner Kent and Chris Kent. In the film Superman II, the Phantom Zone criminals also demonstrated telekinesis.
Despite imaginary stories, and humorous remarks (like the Man of Steel, Woman of Kleenex essay) about the mechanics of breeding between Kryptonian and other humanoid races, Kryptonian DNA is complex enough to make them unable to conceive with most of the races in the universe, thus including humans. The only notable exception is represented by the native Daxamite population, a race of non-powered people found by the Kryptonian explorers later known as Daxamites themselves, upon relocating on the planet and co-inhabiting it peacefully with the natives. The Kryptonian explorers found themselves able to breed, and conceive viable offspring with the Native Daxamite, thus leading to the birth of a new, Kryptonian-like race with only minor differences from the parent race, including the ability to breed with a larger number of humanoid races, including human beings from the planet Earth. Is still unknown if other races may exhibit the same degree of compatibility of the Native Daxamites 
Super powered Kryptonians are vulnerable to kryptonite, radioactive remnants of Krypton, and magic.