London sewers

The London sewers were a sewer system beneath the city of London.

History
Throughout the 19th and 20th century they were used by a variety of species, usually not for their intended purposes. In 1889 they were used as a base of operations by Magnus Greel, who populated areas around his base with his giant rats. (TV: The Talons of Weng-Chiang) The Fifth Doctor later went on a hunting expedition to kill off the ones that remained. (PROSE: A Victorian Interlude)

In the late 20th century, the Cybermen utilised the sewers to travel throughout London prior to a failed invasion of Earth. (TV: The Invasion) They used the sewers again in 1985, by which time they had upgraded themselves and established a base. They apparently used human sewer workers as raw materials for cyber-conversion. (TV: The Invasion, Attack of the Cybermen)

Gryffen Manor was located directly above a particular section of the sewers. Darius Pike made a hideout in the sewers, stashed with supplies in times of emergencies. The K9 Unit which he became a member of, sometimes had encounters in the sewers. (TV: The Bounty Hunter)

During the 2015 Hyperion invasion of Earth, the Twelfth Doctor, Clara Oswald and Sam fled into the sewers to escape from Zraa-Korr. (COMIC: The Hyperion Empire)

After the 22nd century Dalek invasion, alligators lived in the sewers. (TV: The Dalek Invasion of Earth)

In 2254, after stealing a Dalek hoverbout, the Seventh Doctor entered the London sewers to free humans who had been imprisoned there, and had to dodge and defeat slime drops while avoiding walls and spikes. To escape, he battled either a Terrorkon or a Horrorkon, losing the hoverbout but gaining a bomb. (GAME: Dalek Attack)