A Federation space station near Sherman's Planet, overrun by tribbles in the original series episode The Trouble with Tribbles.
Deep Space Station K-7 was a Federation space station close to the disputed Sherman's Planet. In The Trouble with Tribbles, the Enterprise is called there to guard a store of quadrotriticale grain meant for the planet, under the fussy official Nilz Baris.
A Klingon ship under Captain Koloth arrives and its crew is given shore leave on the station, which leads to a bar fight with the Enterprise crew. The trader Cyrano Jones sells tribbles there, and they breed out of control on the station and the Enterprise. When the tribbles get into the grain, Kirk finds most of them dead, revealing that the grain was poisoned. A tribble's hostile reaction to Baris's assistant, Arne Darvin, exposes him as a Klingon agent.
In the Deep Space Nine episode Trials and Tribble-ations, Sisko's crew travels back to K-7 in 2268 to stop Darvin from killing Kirk with a bomb hidden in a tribble.