Arret was the formerly inhabited planet in an unnamed star system. In the ancient past, this was the homeworld of a warp-capable humanoid species that went on to colonize the Milky Way Galaxy. (TOS: "Return to Tomorrow")
In 2293, the location of Arret in the Milky Way Galaxy was labeled in the star chart "The Explored Galaxy", which was on display in James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
Originally Earth-like, though much older, the atmosphere of Arret was ripped away by a cataclysmic war fought 500,000 years before the USS Enterprise arrived in orbit. The ship's sensors detected no life on the planet's surface.
Eleven survivors of that great war had created a shelter 112.37 miles below the surface. This vault was constructed of a material harder and stronger than anything Commander Spock had measured at that time; its atmosphere was "a fraction richer in oxygen than usual".
Originally humanoid, the eleven surviving members of Sargon's species placed the essence of their minds into spheroid receptacles. In 2268, only two spheres besides Sargon's still contained a living mind: that of Thalassa, Sargon's wife, among the five spheres kept on the lower tier, and that of Henoch, a member of the conflict's "other side", on the upper. (TOS: "Return to Tomorrow")