- You may be looking for the Galaxy 4 episode, Air Lock.
An airlock was a device that permitted the passage of people and objects between two drastically different environments, such as between a pressurised space and a vacuum or between an underwater and dry environment.
References
The Aridians' city had airlocks. After the seas dried up, the Aridians would entomb the Mire Beasts by blowing up the airlocks. (TV: The Chase)
The Drahvin leader Maaga tried to kill Steven Taylor by leaving him in a de-pressurised airlock. (TV: Galaxy 4)
To prevent Kirksen from blackmailing the First Doctor into returning to Kembel, and the Daleks, Katarina sacrificed herself by opening the airlock door on Mavic Chen's spaceship, Spar, expelling Kirksen and herself into the vacuum of space and killing them both almost instantly. (TV: The Daleks' Master Plan)
Varga allowed the Second Doctor to enter his spacecraft's airlock, then de-pressurised it until the Doctor agreed to answer his questions. (TV: The Ice Warriors)
The Doctor's TARDIS had an airlock which opened on the roof of the police box. (COMIC: The Underwater Robot, Peril at 60 Fathoms) However, on another occasion the Doctor claimed that the TARDIS didn't have an airlock. (PROSE: When Starlight Grows Cold)
The Kleptons' underwater city had an airlock through which they entered and exited in their flying machines. (COMIC: The Klepton Parasites)
In the 2410s, Darzil Carlisle was orphaned at the age of three when his parents were killed in a freak airlock accident, resulting in the Martian-born child being relocated to an orphanage in Finchley, north London. (AUDIO: The Game)
The Game Station had airlocks. In 200,100, the Daleks forced open the airlock at Floor 494 to gain entry to the space station. (TV: The Parting of the Ways)