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"We all float down here! Hehehe…"

"Pennywise the Clown" IT Stephen King

Basic Information

A sewer is an underground pipe or tunnel used for the removal of human waste. The waste in question may consist of sanitary waste from latrines (called sewage or blackwater), typically blended with waste water from other sources such as washing (called greywater or ullage) and may include food waste and similar things as well. Industrial sewers may also contain trade effluents such as chemical waste or (in case of slaughterhouses) blood. Some sewerage systems also handle surface water run-off rather than having a separate drainage system - this can be useful in that it ensures that the system is properly flushed and diluted, but runs the risk of sewage backing up during heavy rain. Certainly any sewer that doesn't have sufficient flow will block quite quickly.

The development of sewers is a key technology for making urban life healthy - or indeed viable at all in some cases - discarding sewage and similar wastes into surface water courses, or even by surface dumping greatly limits any attempts at sanitation and is likely to have poor health outcomes for the population. That said, most primitive sewers serve mainly to get waste off the streets and do indeed discharge into a major river or other surface water. A more advanced sewerage system should discharge to some kind of treatment facility - which, ironically, may include biological digestion systems very similar to a natural swamp.

Actual sewer design will depend on the technology available to the civilisation constructing it - and on the environment in which it is built. Primitive systems are likely to use existing streams, ditches and suchlike and simply cover them over to convert them into a closed sewer - more advanced ones are likely to use purpose built tunnels and pipework, which may be quite elaborate. That said, the amount of tunnelling should be proportionate to the need - too much and you get the absurdly spacious sewer beloved of fiction but not much found in reality.

Given the risks of sewer blockages, it will generally be necessary for them to be accessible, at least in part, for unblocking work. People may also access sewers to move about covertly - again, this is popular in fiction, but can be hazardous in real life. Quite apart from the fact that sewage is inherently hazardous stuff, it also tends to evolve toxic, explosive or at least non-life supporting gases as it decays and any blockage can quickly lead to a backup of liquid capable of blocking a tunnel to the roof. In the modern era, a particularly popular form of blockage is the fatberg - that is, a mass of congealed fat and grease, originally discharged as liquid waste from kitchens, which has accreted in the sewer, often mixing with other solid wastes as it does so. Londons Victorian sewer system has been shown to develop fatbergs five or six feet high, creating a full or near full height blockage in a walkway access sewer. Obviously the amount of water … and other things … that then build up behind the blockage make clearing it even more dangerous. Access may be easier where the sewers tie into storm drains and/or underground rivers, but these may also support dangerous animals. Especially in those sewers which outfall from that slaughterhouse district.


1. full source reference

Game and Story Use

  • Feel free to introduce PCs that insist on sewer crawling to the full range of hazards - floods, hazardous gases, rats, the lot.
    • Fantasy sewers are particularly entertaining due to the wider range of creatures and the supply of magical process waste available to mutate them into even more hazardous forms.
  • That said, breaking into (or out of) a city via the sewers is very traditional - as noted, primitive examples often outfall in fairly accessible locations. In the modern era, you are probably better off using the storm drains.
  • In the modern era, sewer fighting is regarded as fairly extreme, even by urban warfare standards.
    • Bear in mind that if you discharge an unsuppressed firearm in most sewers, no-one will hear properly for days afterwards…
  • Of course, roaming about in sewers is a traditional form of starter dungeon - for some reason, such sewers are often easily accessible from pub cellars…
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