The sewers in fictional worlds are often an interconnected maze of tunnels and subterranean chambers. Fictional sewers can be both an area to explore (or game dungeon), and a means by which to travel covertly between locations. They can contain secret chambers, hideouts, monster dens, and cult lairs. They may lead to ancient subterranean ruins, cave systems, or underground rivers. They may act as a hidden passageway that connects basements, cellars, or the alleyways. Sewers are often home to rats, roaches, mutants, cultists, monsters, and other dangers.

Fictional sewer systems are often depicted as a series of tunnels containing a central canal-like waterway full of sewage lined on each side by narrow walkways. In some cases there may be no walkways above the grimy water and characters may be forced to wade through the disease-ridden muck. There may be catwalks over certain areas, or makeshift bridges that allow passage from one dry spot to another. Often there are waterfalls of sewage that cascade into large cistern-like chambers. Flooded chambers may be common, where the only way through is to swim. Pipework may line the ceiling overhead. Occasionally there may be very large fans.

Sewers may carry waste or water runoff. Generally these systems are separate.

Fictional sewers often connect to the surface, with access points commonly found on streets, alleyways, and the cellars of abandoned homes. If the sewer contains fresh water, wells are another potential access point. Sewers may deposit their flow (and have additional entrances/exits) on rivers, lakes, or coasts. The sewers of superhero worlds (or any world were heists are common) may serendipitously connect to the employee bathroom of the bank, to the treasure vault of the traders guild, or to the secured archive of the magical library.

Underground areas in worldbuilding are an opportunity to explore the forgotten past, so sewers may lead to disused subway stations, buried ruins, old mines, catacombsCatacombs, or old thieves’ (or bootlegger’s) passageways. Connections to expansive cavern networks, underground rivers, and underground ruins is common. The fictional sewers of fantasy worlds may connect to entire subterranean kingdoms.