Why don't adventurers (and monsters) suffocate in lower levels of dungeons?

Gases in mines and dungeons can be unbreathable

I asked a question about this on Physics to check my reasoning. Why does Carbon Dioxide not sink in air if other dense gases do? An answer states that CO2, which is a product of exhalation, will sink to lower levels in enclosed spaces. There is also a hazard in mines generally from gases seeping out of the rocks.

A while ago I had a guided underground tour in a coalmine. We were warned not to go down a particular stairway because breathing apparatus would be necessary and we would simply keel over and die. It is well-known that miners used to carry caged canaries to detect such gases.


How do PCs and NPCs survive in such conditions? If there is some form of ventilation to the outside world, how come explorers can't avoid the lesser treasures and easier monsters to get to the good stuff by climbing down the ventilation shafts (and escaping the same way)?

  • 27
    $\begingroup$ The magic that creates the monsters, comes with oxygen batteries included. MOst of those monsters are just there. no society, no evidence of longterm inhabitation. The just appear, shortly before you arrive. And they appear with air, food, and no need for sewer systems. I'd rather worry about the 75 tons of Ogre manure that is missing, than the 2 weeks of Ogre breath that is not fouling the air. $\endgroup$
    – PcMan
    Feb 1, 2021 at 0:41
  • 3
    $\begingroup$ We have massive deep underground coal fires burning today that must have significant oxygen ventilation to sustain. But that ventilation is not a simple shaft and is not traversable. $\endgroup$
    – BowlOfRed
    Feb 1, 2021 at 1:43
  • 2
    $\begingroup$ The problem with going down that stairway is probably not the CO2 or whatever sinking, it's much more likely to be outgassing from the coal. $\endgroup$
    – jamesqf
    Feb 1, 2021 at 4:43
  • 1
    $\begingroup$ @CodeswithHammer You don't mean methane? CO2 is already 'fully burnt' coal so it's unlikely it will combust without strong oxidizers. $\endgroup$ Feb 1, 2021 at 16:59
  • 1
    $\begingroup$ @jamesqf: as there is no record in the (reported) history of the dwarves that they ever delved deep enough to dig all the way through to the back side of Discworld I am forced to conclude that either A) it's not possible, or B) it's not probable, or C) it's neither possible nor impossible; it's quantum, or D) they did but they're not telling us; they're sneaky like that, the little devils, or E) they did but no one recorded it, or F) ask Hex. BURSAAAAAAAAAAAR...!!!!!!!!!!!! $\endgroup$ Feb 3, 2021 at 18:18

13 Answers 13


Black Sun Core:

If we're talking monsters and adventurers, we're talking fantasy and magic. I've always ascribed to the core radiance theory in such worlds. The core of the planet generates a radiation (call it "black-light") that can penetrate semi-liquid rock, but is gradually blocked by solid rock/minerals. A fungus-like organism can efficiently capture this radiance, carrying out chemical reactions similar to photosynthesis. radiotrophic fungi are real organisms (thank you Jan Dorniak for the link). The "black light" mushrooms form the basis of the underground food chain. The closer you get to the core, the stronger the black-light, and the more oxygen/less CO2. You thus go a long way to explaining the weird inverted ecology of fantasy worlds.

  • 5
    $\begingroup$ If it’s magical enough then ‘light’ which travels through solids/gases with different opaquenesses makes a lot of sense. If photosynthesising (obscurasynthesising?) such radiation also emits a dim Eldritch glow as a byproduct then you get even more use out of it! $\endgroup$
    – Joe Bloggs
    Jan 31, 2021 at 16:59
  • 1
    $\begingroup$ @Joe Bloggs It is really a useful principle, alluded to but never formally stated in classical AD&D. I tried to make it as neutral as possible, but you could use it in endless ways. It's radiance/impact could generate/act like gravity, for example, so there could be a "Hollow Earth," dark, except lit by a black sun, and low/zero-gravity caves if you go deep enough. The pressure of this light maintains the hollow space, while conventional gravity hold the planet together. There could be heat, like an underground hell, or a mirror surface. Endless uses/variations. $\endgroup$
    – DWKraus
    Jan 31, 2021 at 17:08
  • 1
    $\begingroup$ @ESL the connection to atomic (radiant and particle) radiation didn't escape my notice when I answered. I wasn't sure if there would be enough from under ground to support life while not killing everything else, plus the penetration through rock. But here in Minnesota, we get plenty of radon in basements. $\endgroup$
    – DWKraus
    Feb 1, 2021 at 6:17
  • 1
    $\begingroup$ Since no one posted the link yet, Radiothropic fungi do exist. There's fungi growing literally inside the Chernobyl reactor. $\endgroup$
    – jaskij
    Feb 1, 2021 at 18:10
  • 2
    $\begingroup$ This also explains darkvision. You're not seeing without light, you're seeing by a different kind of light. $\endgroup$
    – Bobson
    Feb 3, 2021 at 20:18

Sad But True...

RPGs in general have long perpetrated the image of wealthy loot to be had for the taking in the world's understructure, D&D has historically been the worst of the perpetrators! Advertised in pubs and broadsides all around the semicivilised lands, dungeons seem to be ready made environments built to entice novice & experienced adventurers alike into the narrative of "good vs evil" -- Elf vs Orc, Man vs Troll, Everyone vs Dragonborn.

But how many of those greedy Elvish or Mannish adventurers ever consider the poor Orclings and lame Trolls slaving and labouring away, unseen, unknown, unconsidered, behind the scenes for their entertainment?

Of course! These adventurers meet the best Orc warriors the Master of the Dungeon has to send against them, but they are considered important to the whole enterprise only so long as, whilst spilling their life's blood far away from starlight and Moon shine, they also spill some hard won GOLD and perhaps some curious trinket or cool weapon. Once the battle is won and the chamber is secured, does the Party care for the slain Orcs? Not on your life! Do any of their noble Elvish clerics ever condescend to heal the Orcs' wounds or even ease their passage into the next life? Nope! They're just broken and now useless junk, furniture & empty chests to be abandonned as the Party moves ever onward and downward...

It's about time the adventuring world gets to know the economic toll and social slavery they perpetrate on Orcish & Trollish communities! Sure, those Orc warriors are paid a pittance by the Master and propped up by promises of freedom & wealth should they destroy the Party -- but when does that ever happen, knowing that the Master controls the Game? In far worse a state are the poor service Orclings and other denizens of the underworld not fit for duty on the frontlines.

Who do you think mucks up after the vampire bats and shovels all the wyrm shit? Who corrals the slimes and makes sure the liche warriors' bones are polished? Who prepares all the finnicky traps for the treasure chests and who cooks for and cleans up after the Orc hordes?

You guessed it ... SLAVES TO THE SLAVES!

Our focus today is one of the sorriest of the sorry. In a better, cleaner world, he might be called an "environmental technician". In a more ideal world, society might honour him for the hard & necessary work he does, and he might find deep in his heart a satisfaction that his work is meaningful and done with love for others. But the Dungeon is not an ideal world and the Master cares not one whit for the poor Treadmill Troll that labours endless day and perpetual night running the titanic exhaust fans that that draw in fresh air from the world above and drives out the stale air from the world below.

Too dim to pick up and swing a cudgel, these poor Troll rejects are forced into a gigantic system of iron cages where they are shackled to the device they are doomed to operate. Some tread to the point of exhaustion upon great sets of bellows, whilst others plod endlessly around a giant winch, and still others find no rest in the windlass.

Poor food, poorer sanitation, little opportunity for rest, no contact with others, no care for callouses, raw skin, wounds or even the chronic pain & injuries associated with ceaseless labour; these Trolls' lives is nothing but unremitting and unrelieved back breaking, foot numbing, leg splintering, and agonising work. Once in the cage, the Troll works until he dies, his only outside contacts are the prods of the Overseers and the slop slingers who toss in their meals of old gnawed bones and gritty, poorly baked bread.

Next time your adventuring Party crosses a dark corridor and your torches are blown out by a "magical wind" or you wonder where the slight but constant breeze in the Dungeon comes from, do remember the poor Treadmill Troll whose climb on the windlass turns the hundreds of belt driven fans that force the air to circulate through the endless miles of service tunnels, hundreds of hidden vents, shafts and windways of the Dungeon!

  • 3
    $\begingroup$ I logged in just to upvote this $\endgroup$ Feb 1, 2021 at 11:28
  • 2
    $\begingroup$ This is an awesome piece of writing. It explains the origins of Mr Nutt, among other beings. $\endgroup$ Feb 1, 2021 at 14:24

There is a draft.


source for image

Deep in the dungeons there are portals - dimensional rifts to elsewhere. Usually the atmospheric pressure is different elsewhere and this generates strong drafts through the portal and air movement through the dungeon. Other things can traverse the portals as well.

If portals are too weird, consider that caves breathe. Real caves like Wind Cave have large air spaces within and these equilibrate with the outside air. A dungeon which connected to a vast space in the inner earth would likewise equilibrate with the outside world depending on their relative pressures. As usual, Lovecraft does it right!

As I walked cautiously in the blackness, the draught grew stronger and more offensive, till at length I could regard it as nothing less than a tangible stream of detestable vapour pouring out of some aperture like the smoke of the genie from the fisherman’s jar in the Eastern tale... The more I reflected on the nature of this cavern wind, the greater my sense of disquiet became; for although despite its odour I had sought its source as at least an indirect clue to the outer world, I now saw plainly that this foul emanation could have no admixture or connexion whatsoever with the clean air of the Libyan Desert, but must be essentially a thing vomited from sinister gulfs still lower down. I had, then, been walking in the wrong direction!

Under the Pyramids, H.P. Lovecraft

  • $\begingroup$ Lovecraft online, how splendid! $\endgroup$
    – Peter Wone
    Feb 2, 2021 at 11:19

Certain soils are the world's best carbon sinks

Made naturally from decaying vegetation or organic matter; peat / turf is a carbon sink:

The peatland ecosystem covers 3.7 million square kilometres and is the most efficient carbon sink on the planet

... and ...

Globally, it even stores up to 550 gigatonnes of carbon, representing 42% of all soil carbon and exceeds the carbon stored in all other vegetation types, including the world's forests

Things die in the dungeon, fall to the bottom. They decay. And eventually fertilise the bottom, turning it into rich soil. Peat grows down underground at a rate of about 1mm/yr. After a few years, the peat bog is sucking CO2 out of the bottom level of the dungeon fast enough to keep it breathable.

  • 15
    $\begingroup$ To clarify, peat bogs absorb CO2 by growing as green plants on the surface. The carbon sink is not because the peat decays but because it does not decay, accumulating over thousands of years and, perhaps, even becoming lignite and coal. A dungeon needs to have continuous light such as from some clean magical source to make use of this. $\endgroup$ Jan 31, 2021 at 22:25

Your main question is about why adventurers dont climb up and down the ventilation shafts to get to the treasure, circumventing the whole dungeon. You could actually use that question in our current world: why dont thieves and burglars climb down our current ventilation systems to bypass our doors and locks?

The answer is equally simple: the ventilation shafts are too small to pass through. These dungeons can have a network of ventilation shafts that connect to the dungeon and ventilate it. Naturally any dungeon still functioning would have enough ventilation while any dungeon not functioning would have stopped being build any farther than its ventilation allows.

The ventilation shafts can be small and come from far away, similar to some ant nests where the ventilation shafts can be extremely far from the main entrances.

Should you want more reasons to bolster the above idea you can look into semi- fantastical solutions: non-photosynthetic plants and a symbiosis with ants. Non-photosynthetic plants are our current day cave-plants and come in a variety of forms and use chemical reactions for their food. Many will oxydize things like iron, sulfur or nitrate in their environment, the oxygen they take from the carbon dioxide that plagues your dungeon. You can have a layer of small ventilation shafts beneath the lowest dungeon floors riddled with these types of plants that will convert all the carbon dioxide. Many wont generate oxygen, but will instead take CO2 out of the air and create space for oxygen to come through.

Many creatures in fantasy dungeons have evolved to actively live in them, so its a small leap to say that some ants have learned to live there as well. Ants are not unknown to actively farm plants and other insects for food or other benefits. In this case the ants are interested in not suffocating by CO2, so they inhabit the ventilation shafts and tend to the plants so there's both enough and it doesnt clog the ventilation shafts. These same ants can carry things like iron (from gear of monsters and adventurers) and sulfur to these plants, as well as live of the detritus and corpses of the denizens of the dungeon. They could even be the main source of ventilationshafts to any dungeon, creating thousands of small tunnels to keep each room supplied.

  • $\begingroup$ A ventilation shaft for something as big as an underground dungeon will necessarily be very big. your main options are to split it into many flues, or to install many steel grates over it, with guards to alert the other monsters in the dungeon should an adventurer try to cut their way through the ventilation shaft. $\endgroup$
    – Globin347
    Feb 1, 2021 at 13:38
  • $\begingroup$ @Globin347 a big one might be more efficient per volume but having many small ones is also possible, with the added advantage that you can tailor the airvents to more area's since you dont have as efficient active machine-powered ventilation like we do. $\endgroup$
    – Demigan
    Feb 1, 2021 at 14:04
  • $\begingroup$ @Globin347: the amount of air the ventilation shafts need to handle doesn't scale with the size of the dungeon, it scales with the size of its long-term breathing population. A huge dungeon with only a few monsters (or monsters that hibernate) and only rare visits from adventurers doesn't need all that much ventilation. $\endgroup$ Feb 1, 2021 at 21:51

Either the denizens use a breathing apparatus, or the dungeon is designed with air flow in mind.

Here's a similar question: How do termites not suffocate in their underground dens? The answer lies in termite mound construction:



If you want to be a bit more realistic, given the scale, you might add a central chimney that stretches from the very deepest chamber to the very tippy top. Air must be constantly flowing up through this chimney, and new air must be pulled in through ventilation shafts. I can think of three main ways to keep the air flowing:

  1. Light an enormous fire at the bottom of the chimney. It would have to be the size of your average house at least; keeping it lit would require far more fuel than could be realistically gathered, except by a highly advanced and massive resource gathering infrastructure (whether that be magic or technological.) If this is the case, this dungeon will be more like a city than a dungeon, and the enormous pillar of smoke would be visible for miles on the surface.

  2. alternatively, it might be more plausible to place large fans inside of the chimney shaft. Powering these would probably be less fuel intensive than keeping a giant fire, however you go about doing that. Placing smaller fans in the air inlets would help increase efficiency.

  3. Use a magic air current. In practice, this probably works the same way as the giant fans.

Here is another similar question about Dwarven underground cities:

Giving Tolkien Architecture a Reality Check: Dwarvish Kingdoms

  • 3
    $\begingroup$ you don't need a fire or fans, if the chimney is tall enough you can build an wind tower/ wind catcher. $\endgroup$
    – John
    Jan 31, 2021 at 18:13
  • 3
    $\begingroup$ Ooor: the bottom of the chimney is the hoard of a Dragon. Dragon has an easy exit/ingress, and their natural body heat (which I imagine is warm) has heated their Hoard to the point where it creates an updraft. $\endgroup$
    – Joe Bloggs
    Jan 31, 2021 at 21:40
  • $\begingroup$ "light a fire" and call it a dragon $\endgroup$
    – WernerCD
    Feb 1, 2021 at 8:29
  • 1
    $\begingroup$ Also: someone built these dungeons (in most cases). And nobody would build a basement they'd suffocate in, meaning that they DID just plan for it $\endgroup$
    – Hobbamok
    Feb 1, 2021 at 9:51
  • 1
    $\begingroup$ The simple perpetual source of fuel could be a gas seep - the constant flame is needed to keep the dungeon from filling up with natural gas (see the Darvaza gas crater in Turkmenistan - burning continuously since 1971). Setting a fire at the bottom of a chimney was common mining practice before mechanical ventilation. The fire can be fairly small as it only needs to get the stack effect started - any temperature difference would work so long as the heat went up the chimney enough to get it started, then it would mostly perpetuate itself. $\endgroup$ Feb 2, 2021 at 15:55

A side effect of dragonfire and cookpots.

Assume that somewhere down in the dungeon, there is a kitchen with a chimney. Or a dragon with a chimney. Warm air rises through the chimney and sucks more air along through other ventilation openings. If the air ducts are right, the fire doesn't even have to be all that deep down.

This is similar in principle to a steam ejector, and the principle has been used for centuries in siege engineering.


An answer states that CO2, which is a product of exhalation, will sink to lower levels in enclosed spaces. There is also a hazard in mines generally from gases seeping out of the rocks.
How do PCs and NPCs survive in such conditions?

The dungeons exist inside a vast underground network of natural caves. The cave walls are mainly composed of very porous limestone. The whole environment in the caves has a lot of mist and water vapor.

CO2 will dissolve into the misty water and form carbonic acid. That acid will react with the limestone and the CO2 will be sequestered in the rocks.

The interior of the dungeons contain a huge volume of tunnels and lime dust. The surface area of the exposed rock is so great that there is no concern about exhausting its ability to sequester more CO2.

Additionally, since the caves go very deep, most of the CO2 has sunk so far down that most adventurers won't reach that point anyways.

If there is some form of ventilation to the outside world, how come explorers can't avoid the lesser treasures and easier monsters to get to the good stuff by climbing down the ventilation shafts (and escaping the same way)?

The great part about sequestering the CO2 in the cave walls and dust is that you don't need any kind of ventilation shafts.

It is well-known that miners used to carry caged canaries to detect such gases.

As for other gasses, there may yet be some passageways that are deadly to enter. The seasoned adventurer will note that the tunnel is littered with the bones of his predecessors and decide to go another way.


The physics forum gave good answers, but they were technical, so I'll try to keep this short. Molecules of CO2 are a bit heavier than O2, but heat jostles them around and any circulation of air mixes them. It is possible to have a layer of CO2 under air, if you fill a box with it from underneath carefully as is done to euthanize animals, but it isn't really stable. Think of a black and tan - it can be done, but it's not the first thing you think would happen if you mix two kinds of beer together. It is easier if you chill the CO2 - every few years someone manages to die at a hot tub party decorated with the spooky emanation of vapor from dry ice dropped to the bottom.

Mines (or dungeons) can certainly have a source of CO2 beneath that gradually fills large parts with unbreathable gas. Sealed rooms full of people (or monsters) will accumulate some CO2 as the air is used up (with a little chemistry you can calculate the rate that happens based on the calories of stored fat they burn and the moles of oxygen needed to react). But the average small cave "probably" doesn't have that problem, and if it did have that problem, people would have noticed it when they were cementing the spikes onto the bottom of that first pit. Deep mines or construction would be expected to have good air shafts (or preferably, magic gateways to a nice beach for worker R&R), also motivated by the need to dissipate the choking dust from excavation and, for especially virtuous dungeon owners, a desire to allow employees to escape if there is an accident.

  • $\begingroup$ "it can be done, but it's not the first thing you think would happen" -- if I ever open a bar, that's the description I'll put in the menu for B&T. $\endgroup$
    – Tom
    Jan 31, 2021 at 20:56

The monsters are not actual monsters, though they look like it, but spontaneous emanations of the very nature of the dungeon. They do not need to breathe any more than they need to eat.

They are there to lure adventurers to battle, and the adventurers are also provided clean air, so as to enable the nefarious purposes of the dungeon.


The answer would depend on the backstory of the dungeon itself. Dwarven dungeons in LOTR for example are engineered and built, including air passages. Many of the dwarven dungeons in LOTR are actually built into mountains, which would make designing such natural airflow a lot easier than if you were to just build a huge dungeon straight down.

Stygian Abyss in the Ultima series (as seen in Ultima Underworld) is similar with it's own underground rivers and such on the upper levels, but being just magical on the lower levels. If the dungeon is connected to natural underground caverns that have underground rivers running through the area that could be used as reasoning for a source of fresh air (from wherever the rivers flow from).

If you want to keep consistency and not just rely on a "wizard did it" explanation an easy cop out is having the dungeon built into a mountainside. Probably a lot of the mines were built in mountains in ancient times for similar reasons.


Exotic Monsters

If your monsters inhale CO2* and exhale O2, like plants, then your adventurers should have no issues breathing. The air will be musty and dank (and not in the sense related to a certain plant which is ignited and inhaled), but perfectly breathable.

This actually creates a unique and different problem: a dungeon with no monsters can be more dangerous than one with monsters, creating an interesting situation where adventurers may prefer nonlethal weaponry in order to avoid suffocation.

* or any other hazardous gases.


We have lots of geothermal heat so we just need to build dungeon to use chimney effect for ventilation - build upcast shafts in mountains and they will draw air out.

And smart hero with parachute may use a shaft like this to escape dungeon. :-)


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .