The Phantom Zone was a hellish prison discovered by Jor-El for Kryptonian criminals. The Zone existed outside of space-time and was layered beneath the universe of Prime Earth, found in the Bleed in between Apokolips and Dream. The prison caused aging to cease and pain to be endless, transforming it's prisoners into bluish phantoms. Eventually, Superman began to use it to trap villains of his own, including Mongul and the denizens of Warworld. Inside the Phantom Zone existed another supesecret zone designed to contain Doomsday known as the Forbidden Zone.
The Phantom Zone was not all uncharted, however, as since most of it was vast empty unknown space, parts of it actually belonged to a "civilized" afterlife in which Hades, the realm of Pluto, the Erishkagal and the Land of No Return, presumably the edge of the zone, are located. These afterlives were known as the Underworld. Once given to the hands of Michael Holt, Oliver Queen and Ray Palmer, the Phantom Zone was contained into a more manageable pocket universe. Palmer then devised a method of using the Phantom Zone to travel through the depths of space. It was then used to contain certain prisoners of the Earth 2 War.