The Sewers of Sanctuary were once the crown jewels (or at least the shining anklets) of a pristine and beautiful city. Senturae had a network of sewers running under the entire city, with most sewers 6-8 foot tall. Penjarre sank the land, putting many of the sewers underwater. However, it was the moral descent of the denizens of Sanctuary over the last three hundred years that is responsible for the current reputation.
The sewers no longer connect everywhere across the city. There are a series of small networks, from a single tunnel length, to the warren of filth and depravity that lies under the Maze. The larger networks are used by forgotten cults, drug pushers, or criminals too hated and hunted to walk the streets of the most lawless city in the baronies. Some long tunnels are used for smuggling, avoiding the kick-backs and tarrifs of the Docks. Shorter sections are used as basements, priest holes, or neighbourhood meeting spots for a quiet drink, secret liason, or human sacrifice.
Twice a day some tunnels will rapidly flood with a tidal bore from Confederation Bay. As the sewer configuration changes regularly, you can be confident but never certain that you will not be swept away to become just another body floating under the Docks.
There are many maps and fragments describing the sewers. None can be trusted, and the sewers are always changing, with a new wall added here, or a long-disused connection reopened there.