A sewer was a cavernous tunnel and pipe system through which effluent and excess storm water flowed away from the inhabitants of a city.
In 1889, the Fourth Doctor and Leela were pursued through the London sewers by giant rats. The rats were used by Magnus Greel to guard his underground laboratory. (TV: The Talons of Weng-Chiang)
In November 1930, the Tenth Doctor and Martha were chased through the sewers of New York City by Daleks and pig slaves. (TV: Daleks in Manhattan)
During one of their attempted invasions of Earth, the Cybermen operated in the London sewers. (TV: The Invasion) They would do so again in 1985. (TV: Attack of the Cybermen)
In 1980, the Fourth Doctor and Celia Soames travelled through the sewers of Budapest in order to enter the labyrinth of Buda Castle. (AUDIO: The Labyrinth of Buda Castle)
The Sixth Doctor and Peri Brown investigated the London sewers on two occasions, in 1985 (TV: Attack of the Cybermen) and January 1986. (PROSE: Time Wake)
In the 2000s, Weevils lived in the sewers of Cardiff. (TV: Combat, et. al)
In 2050, K9, Starkey, Jorjie and Darius regularly traversed the sewers of London. (TV: The Bounty Hunter, et. al)
In 2254, the Seventh Doctor and Ace infiltrated a Dalek occupied London through the sewers inhabited by Mutants and a Terrorkon. (GAME: Dalek Attack)
Missy and Clara explored a sewer beneath a Dalek city on Skaro. Missy explained to Clara that the sewers' primary function was not to house the Daleks' waste, but instead to house decomposing, used Dalek mutants. This was because Daleks were genetically engineered to not die naturally.
The Twelfth Doctor later tricked Davros in his own plan. He used his regeneration energy to strengthen the dying Dalek matter and bring it back to full health. The mutants from the sewers then rose up and attacked the overcity. (TV: The Witch's Familiar)