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)
The Sixth Doctor and Peri investigated the London sewers on two occasions. (TV: Attack of the Cybermen, PROSE: Time Wake)
The Seventh Doctor and Ace infiltrated a Dalek invaded London through the Sewers inhabited by Mutants and a Terrorkon (GAME: Dalek Attack)
The Tenth Doctor and Martha were chased through the sewers of New York City by Daleks and pig slaves. (TV: Daleks in Manhattan)
Weevils lived in the sewers of Cardiff. (TV: Combat and others)
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)
K9, Starkey, Jorjie and Darius regularly traversed the sewers of London. (TV: The Bounty Hunter and others)