The Tantalus colony was a Federation penal colony, located on the planet Tantalus V, that served as a prison and hospital during the mid-23rd century. Violent criminals were imprisoned there for rehabilitation.
The facility contained a mix of surface and underground structures; patient housing, therapy, and staff offices were located underground. Above-ground structures included an elevator head, through which one could access the patient facilities as well as other buildings. Like all penal facilities, the Tantalus colony was protected by a force field that inhibited transporter traffic. To visit the planet or transfer cargo, it was necessary to contact Tantalus personnel and request the field be deactivated.
In 2266, Tantalus initially benefited from the leadership of Dr. Tristan Adams, whose theories on penology had revolutionized such facilities, transforming them from cages into "decent, clean hospitals for sick minds." Late in 2266, when Dr. Simon Van Gelder escaped from Tantalus, it was revealed that Adams had been conducting unauthorized and highly unethical experiments on patients and staff alike, subjecting them to an experimental therapy device, the neural neutralizer, in an attempt to learn more about such therapy. Ironically, Adams was killed by the neural neutralizer when an experiment he was conducting on Captain James T. Kirk went awry, and Kirk escaped. After Adams' death, Van Gelder assumed leadership of the facility and destroyed the neutralizer. (TOS: "Dagger of the Mind")
Background information Edit
In the final draft script of "Dagger of the Mind", the exterior of the colony's surface facilities was described as a "squat building" and the script went on to say, "It's small, obviously the superstructure leading to a vast underground colony."
Ignoring this script description, the image used for the colony's surface facilities in the original aired edit of "Dagger of the Mind" was a matte painting which had been used for a massive lithium cracking station on Delta Vega, visible in TOS: "Where No Man Has Gone Before". This made it seem as if the above ground parts of the Tantalus colony were the larger parts of the facility, including power generation, despite the opposite having been scripted.
The remastered version of "Dagger of the Mind" replaced the visual of the colony's surface facilities with a new digital matte painting, which took its cue from the script, featuring only a small building on the surface of Tantalus.