Starbase 12 was a Federation starbase administered by Starfleet. It was located on a planet in the Gamma 400 system, in Sector 25712. It was located close to the planets Achrady VII, Ceti Alpha V, Gemaris V, Mab-Bu VI, Pollux IV, and Risa. (TOS: "Space Seed", "Who Mourns for Adonais?"; TNG: "Captain's Holiday", "Power Play", "Aquiel")
Later that year, after the USS Enterprise had been stopped by an unknown force, Captain Kirk ordered Lieutenant Nyota Uhura to relay the position and circumstances of the starship to Starbase 12. (TOS: "Who Mourns for Adonais?")
In 2364, the starbase was inexplicably evacuated for two days. Starfleet Command gave no explanation for this action. Captains Walker Keel, Rixx, and Tryla Scott saw this as further evidence of a conspiracy within Starfleet. (TNG: "Conspiracy")
In 2369, Starbase 12 sent a message on Starfleet Commnet One via an encoded beta. This base was named on the chart "Sector Ipai Relay Log - Relay Message Flow 1293" in Relay Station 47 Ops. (TNG: "Aquiel", production art)
Later that year, when Lieutenant Aquiel Uhnari and Klingon Commander Morag were believed to be coalescent organisms, the USS Enterprise-D would have ferried these two to a secure medical facility on Starbase 12. (TNG: "Aquiel", okudagram)
Background information Edit
According to Who's Who in Star Trek 2, the Starbase was established to investigate unexplained phenomena in the Gamma 400 system. These phenomena were later discovered to have been the work of Apollo.
According to the Star Trek: Star Charts (p. 62) and the Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Starbase 12 was located in the Beta Quadrant.
Starbase 12 was also featured in the video game Star Trek: Bridge Commander and the novel Ship of the Line, in which the USS Bozeman protected Starbase 12, which was the building site of the USS Enterprise-E. The starbase was also mentioned in Star Trek: Elite Force II.