The sewer map generator draws a stretch of city sewer on five-foot squares, ready for the moment the party drops through a street grate. A main channel of running water crosses the sheet with a stone walkway on either side, and a cross channel meets it at a junction chamber, either as a full crossroads or as a T. Side tunnels lead off the walkways to chambers: the ladder up to a street grate, a cistern, a sluice gate with its wheel, a smugglers' cache, a collapsed tunnel, and the lair of whatever lives down here. Narrow crawlways branch off the walkways and dead-end in the dark. Every area is numbered and keyed.
How the sewer is laid out
Sewers are built, not dug, so the map is regular where a real one would be: channels run straight, walkways are a constant width, and the junction is a square chamber where the water meets. The channels are two squares of water with a square of walkway either side, which leaves room to fight along the edge and a real risk of going in.
The side chambers are placed in the four quarters around the junction, each joined to the nearest walkway by a tunnel. The ladder to the street is always there, because the party needs a way in and out. The other chambers are chosen at random each roll, so one sewer has a smugglers' cache and a sluice, and the next has a cistern and a nest. Crawlways are cut last, off the walkways, never touching anything else.
What's down there
The street grate has its ladder. The cistern is a flooded room with columns holding up the street above. The sluice control has its gate and wheel, which opens a lot of possibilities for flooding the channels. The smugglers' cache is a stack of crates and a lamp. The collapsed tunnel is full of brick rubble. The lair is a nest of filth with bones scattered round it. Pipes along the channel walls drip into the water.
Using it at the table
The sewer is the classic city dungeon: the thieves' guild's back door, the route into a locked building, the place the plague started. A keyed map makes it quick to run. Put a wererat in the lair, a gang in the cache, and give the party a reason to want the sluice open or shut. The water channels make fights interesting: shoving someone in is always an option. It suits D&D 5e, Pathfinder and most d20 games.
Free SVG and PNG downloads
Both formats are free for your games, streams and published adventures. The PNG is a 1920-pixel render for a VTT, and the Print button saves a black-and-white version for a home printer. The SVG is the original vector file: open it in Inkscape, Affinity or Figma to add a grate, flood a walkway or rename the lair. Everything on the map is drawn by our own code, so there's no attribution to carry and no asset licence to read.
What it won't do
It draws one section of a sewer. It doesn't draw the streets above, the whole network of a city, or the buildings the tunnels lead into. For the city above, roll a city map; for a dungeon the sewer breaks into, roll a battle map.
Name the city, then go under it
The city name generator names the city above and draws its streets, the thieves' guild name generator gives you the people who use the cache, and the battle map generator draws the vault the crawlway was dug toward.