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There is a newly discovered dungeon believed to be made up of a series of interconnected tunnels and caverns that runs deep beneath the ground, the place is thriving with all kind of terrifying monsters and they share one common trait that is they likes to fart a lot.

Unlike mammals their flatulence has high concentration of hydrogen sulphide gas which is both flammable and toxic to living things, I am wondering how do they ventilate the air in the dungeon without mechanical parts since they could also suffocate or poisoned for overexposure to the gas.

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  • $\begingroup$ Toxic to.all creatures? Including themselves? $\endgroup$
    – Stilez
    Jan 18, 2021 at 11:08
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    $\begingroup$ @Stilez: yes even these monsters will suffer from asphyxiation (deprive of oxygen) $\endgroup$
    – user6760
    Jan 18, 2021 at 11:12

5 Answers 5

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The caves flourish with sulfur oxydizing bacteria:

Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to produce energy. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy in order to build their structural components, survive, grow and reproduce. Some inorganic forms of reduced sulfur, mainly sulfide (H2S/HS−) and elemental sulfur (S0), can be oxidized by chemolithotrophic sulfur-oxidizing prokaryotes, usually coupled to the reduction of oxygen (O2) or nitrate (NO3−).

Sulfur oxidizing bacteria (SOB) are aerobic, anaerobic or facultative, and most of them are obligate or facultative autotrophs, that can either use carbon dioxide or organic compounds as a source of carbon (mixotrophs).

A common game between the youngsters of the cave dweller is to fart on rocks to see the bacteria growing quickly thanks to the supplied gas. The one who make the biggest growth is the most respected.

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    $\begingroup$ Nice touch with the “game”.... cracked me up 😂 $\endgroup$
    – fartgeek
    Jan 19, 2021 at 2:27
  • $\begingroup$ Note that most of the pathways at the Wikipedia link consume oxygen, so asphyxiation would still be an issue without a supply of fresh air. This answer does at least remove the (rather toxic - when you stop smelling it without getting away from it, you're being poisoned) H2S $\endgroup$
    – Chris H
    Jan 19, 2021 at 10:59
  • $\begingroup$ Over the years, you would get a yellow layer of sulfur over every surface. Highly flammable. $\endgroup$
    – mart
    Jan 19, 2021 at 16:24
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A common tactic in old mines was to set up multiple pairs of shafts. One shaft would have a fire lit at the bottom of it. The hot air in that column would rise, forcing cold air to flow down the paired cold shaft and setting up a flow of air through the mine.

How does this help you?

Well, any way to set up a temperature difference can create this kind of airflow. Sunlight on one half of a mountain (or mirrors to direct sunlight to the base of a shaft if no mountain is suitable), a subterranean river that’s warmer than the air, geothermal activity if your dungeon is big enough, even the body heat of your creatures if they’re clustered in one place; all of these could set up permanent airflow with no moving parts needed!

If the dungeon is designed and set in a location with a prevailing wind direction (say somewhere like a valley in a steep mountain range) then pairs of entrances can be built with open doors facing into/out of the wind to achieve the same effect. The wind blows into one entrance, circulates, then blows out the other. If you don’t want these to be ‘entrances’ per se then you can make small buildings with one wall missing atop a ventilation shaft,

One thing to watch out for with this kind of ventilation is that the setup of the dungeon matters. Opening/closing the wrong sets of doors might lead to chunks of dungeon that are cut off from ventilation, or might just stop it working altogether! The basic requirement is to keep the dungeon split into sections with an ‘in’ at one side and an ‘out’ at the other and to minimise airflow between the sections.

This gives your dungeon a reason to have a bunch of seemingly randomly placed doors, some of which are open, some closed, and some locked.

It also means a rampaging band of adventurers is likely to wreck the ventilation, so watch out for that!

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  • $\begingroup$ Not all mines, though. Where one could expect grisou/firedamp, open flames were forbidden. en.wikipedia.org/wiki/Firedamp $\endgroup$
    – L.Dutch ♦
    Jan 18, 2021 at 9:56
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    $\begingroup$ @L.Dutch: IIRC even firedamp-prone mines used open flames, they just kept them in specific areas well away from any active coal faces, the rationale being that good ventilation would disperse the gas (and also coal dust) to the point where it wouldn’t spontaneously explode by the time it reached an ignition source. The gases produced by these creatures would combust on contact with the flames, but would never reach concentrations high enough to cause explosions. $\endgroup$
    – Joe Bloggs
    Jan 18, 2021 at 12:24
  • $\begingroup$ Hot springs or even an open lava pit at one end. $\endgroup$
    – rtaft
    Jan 19, 2021 at 14:12
  • $\begingroup$ @rtaft Because every evil dungeon lord needs their own lava pit and/or luxury spa. $\endgroup$
    – Joe Bloggs
    Jan 19, 2021 at 21:01
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The monsters have access to an underground stream for water

Towards the bottom of the dungeon, there's a nice flowing stream of water. It's nice and fast - bubbling and splashing and spraying everywhere. The monsters drink from the upstream part of it, and put their waste in the downstream part of it. It flows fast enough that there is no risk of their sewage getting in their drinking water.

Hydrogen Sulphide is highly soluble in water:

The gas is highly soluble in water to the extent of 4000 mg/l at 20°C and one atmosphere

If we assume your monsters are farting 400g of hydrogen sulphide per day each (that's A LOT), only 100 litters of water flow in 24 hours per monster are needed to absorb it.

A small creek could absorb the hydrogen sulphide of hundreds or thousands of monsters. The water will become slightly acidic, but since the monsters wastes go in the water too, no-one downstream drinks it anyway.

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    $\begingroup$ H2S being heavier than air would settle to the bottom of the cave, so flowing water may be a solution, especially when there is also some waterfall adding moisture and bit of air movement along the river. $\endgroup$
    – Congenital Optimist
    Jan 18, 2021 at 21:08
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Honestly gas exchange is going to be a big problem for your system in general. Burrowing animals that build extensive burrows like termites, ants, prairie dogs, and wombats often have problems appropriately ventilating their burrows so they have enough sufficient oxygen. A huge dungeon is going to have even larger problems maintaining an appropriate oxygen balance because it's even deeper underground with fewer openings to the outside than a prairie dog town or termite mound, even if it's larger size means that in the short term oxygen reserves will last longer I would suggest looking at how termites and prairie dogs ventillate their burrows and go from there.

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Oxygen depletion is going to be an issue long before farts are. You need air exchange somehow. Multiple openings and the prevailing winds are probably the right answer as it's passive--it won't stop if the caretaker dies.

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  • $\begingroup$ A cliff side opening on the prevailing wind side is perfect. It's not easy to access by humans, and because it's a cliff the wind gets forced around the cliff and through the opening. It could also make a great scene for an emergency escape from the dungeon. $\endgroup$
    – rtaft
    Jan 19, 2021 at 14:18

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