In 1961, The Phantom Superboy was a story that introduced the Phantom Zone. Superboy discovered that the Phantom Zone was a strange twilight dimension that existed on a different plane of existence. Krypton used it as a prison for their most horrible criminals. There was no death penalty on Krypton, so sending them to the Phantom Zone, a place discovered by Supermans father Jor-El, was seen as the worthy alternative. A press of the button on the Phantom Zone Projector and the criminal was exiled into the other dimension, not to be retrieved until their sentence was up (prisoners could be observed via special viewscreens and retrieved by the same projector).
In this place, the prisoners existed as spirits, unable to age, but also unable to touch each other or interact with the true physical universe, which they could still see and observe. Curious that Kryptonians considered death too harsh, yet one could argue that being exiled to existence as a ghost for anywhere from a few years to a few decades, forced to watch friends, family and enemies go on with their lives and not able to interact at all, would be a fate far worse than death.