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. (DW: The Talons of Weng-Chiang) The Doctor later went on a hunting expedition to kill off the ones that remained. (DWM: A Victorian Interlude)

In the late 20th century, the Cybermen utilised the sewers to travel throughout London prior to a failed invasion of Earth. (DW: 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. (DW: The Invasion, Attack of the Cybermen)

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