London sewers

The London sewers were a series of tunnels beneath the city of London on Earth.

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