Although official propaganda indicated that the colony was for insane individuals, Tantalus was really a prison for those who had fallen out of favor or refused the will of the empire. Those individuals sent to Tantalus were ones whose family had some limited influence within the bureaucracy of the empire and yet could not be killed outright. Upon arriving at the planet, inmates had their belongings confiscated and were subjected to torture as well as the neural neutralizer.
In 2264, Lieutenant Commander James T. Kirk visited the colony posing as a doctor. Kirk was seeking a favor from his friend Dr. Simon Van Gelder, a clean psychiatric evaluation. However, Van Gelder was seeking to leave the planet, believing Dr. Adams to be misguided in his intentions to reform the colony's prison population. As part of his plan to escape Tantalus, Van Gelder released all of the inmates and murdered Dr. Adams.
During the confusion, Van Gelder, Kirk and Marlena Moreau went down into the colony's vault where all of the personal items confiscated from the inmates were stored. It was in the vault that Kirk discovered the Tantalus field and tested it on Van Gelder, killing him. (TOS - Mirror Universe short story: "The Greater Good")