Sewer

A sewer was a cavernous tunnel and pipe system through which effluent and excess storm water flowed away from the inhabitants of a city.

On Earth

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)

In November 1940, Ace were chased through the sewers of London City by Cybermats. (PROSE: Illegal Alien)

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)

Elsewhere

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)

The sewers of the planet Tersurus boasted an incredibly complex design, one that it had taken the Master 936 years to escape from, each time he'd been knocked in taking 312 years to escape. (TV: The Curse of Fatal Death)

On the planet of Lobos, Yasmin, with the help of the loba leader of the resistance Pry, used the sewer system under Kanda City to break out the prison. (PROSE: The Good Doctor)