Throughout the long history of the planet Skaro, after the creation of the Daleks, these sewers were made beneath the ground on the planet. As the years went on and the various Daleks grew older and older, the Daleks were left to rot in the sewers, beginning to decay they grew into the walls of the sewers attaching themselves to each other and liquifying until no longer looking like their mutant selves.
After teleporting away from their supposed extermination by the new Daleks in the Dalek City, Clara Oswald and Missy found one of these sewers and began exploring, while Missy told the purpose of the sewers.
When a young Dalek entered the sewer, Missy used her brooch, which had dark star alloy inside it, to poke holes in the Dalek's machine casing, allowing the decaying Daleks to penetrate it and kill it.
Later when the Doctor touched the cables, in fact Colony Sarff, his regeneration energy was drained by the new Daleks and Davros' wheelchair, but also to the angry, decaying Daleks. This gave them new energy-filled life, and allowed them to rise from the sewers to attack the Dalek City and the youthful Daleks in revenge for abandoning them and letting them rot in the sewers. (TV: The Witch's Familiar)