Minecraft wiki entry for Building_a_metropolis, associated text: Sewers No one likes to drink their own filth, so make sewers underground that link all the houses and buildings. You can have the sewers flow into a sewage treatment plant, unless you are building a medieval city, so the filthy water can be cleaned. Use mossy cobblestone or mossy stone bricks as walls and add a water stream. Don't forget to light it, as otherwise, the sewers will become a dungeon. You could also use the sewers as a secret way of transportation and hide entrances in important buildings.