Largescale view of sewers from Bay12 devlog.
Sewers are underground networks of tunnels found below the streets of a town. Most sewer tunnels are partially flooded with water, making exploration possible. Water is sourced from nearby rivers. Sewers with no nearby flowing rivers will be completely dry instead. Towns with sewers have visible floor grates in the middle of streets.
All sewers have at least one Sewer Access, as shown in the fast travel screen. The entrances are simple downward ramps or stairs that directly lead down to a walkable section of the sewers. An alternative entryway involves entering through a flooded tunnel by swimming under the river, but unless you're amphibious or don't require breathing, this method poses a high risk of drowning. Dungeons and catacombs can also connect into sewers.
Items and corpses can be found on the floor. If the world has caverns present, subterranean animal peoples will settle in the sewers. These outcasts will wander around the flooded sections and attack you on sight. They are armed with spears and blowguns. Be careful not to dodge into the water without at least an Adequate swimming skill, lest the fall stun and drown you. Wild animals with
[LARGE_PREDATOR] tags (e.g. alligators) can also take up residence in the sewers.