The Tantalus colony was a Federation penal colony, located on 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 the elevator head through which one accessed 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 Kirk went awry, and Kirk escaped. After Adams' death, Van Gelder assumed leadership of the facility and destroyed the neutralizer.
The image used for the surface facilities was the matte painting used for the lithium cracking station on Delta Vega, seen in TOS: "Where No Man Has Gone Before". It is likely that the above ground parts of the facility were the larger parts of the physical plant, including power generation.