There is an underground city in a multi-level cave with two entrances; it is otherwise enclosed. It's shaped more or less like an inverted pyramid, getting narrower with increasing depth. A fire breaks out at the top level, which contains a wide, open area and is also where the first entrance is. The second entrance is at the bottom. The volume is around 50 million cubic meters, half of it air and half of it cave walls. The fire has quickly grown to a degree that the locals cannot contain it, and they must evacuate.

Most real-life fire safety manuals are written with the assumption that only the ground floor can be used to escape, which is not the case here.

My main questions are: Which entrance (if not both) does the smoke go toward? Which entrance (if not both) does the carbon dioxide go toward? If carbon monoxide is generated, which entrance (if not both) does it go toward? What other byproducts can be expected here?

Details that may affect the answer:

  • Atmospheric pressure is 5x sea level atmosphere, but composition is the same as regular air.
  • The lower part of the cave contains chemosynthetic fungi that recycle carbon dioxide into oxygen with a combined capacity of around 100 tonnes per day. However, these fungi are themselves very flammable, and if the fire spreads downward (which it can) it will quickly consume them.
  • The primary material being burned is not exactly wood but a dried fungus-derived material of similar flammability; although the "wood" is an omnipresent building material in the inhabited areas, the high ceiling of the top layer is bare rock.
  • The upper and lower entrance are about equidistant from the source of the flames.
  • Each entrance has an area of around 10 square meters, which is not enough to evacuate the population in time.
  • $\begingroup$ Research coal fires in mines, and don't forget there are likely ventilation shafts in a large, inhabited cave complex. $\endgroup$
    – DWKraus
    Commented Sep 28, 2020 at 14:47

2 Answers 2


"If unsure. Bottom door!"

On alarm, go to the nearest exit if you can see a safe path to it. Some people will escape out the top before the fire gets to the door, but the fire will probably burn towards the top door initially before it starts building it's own wind system.

Everyone in sight of an exit goes out it. Everyone who doesnt know where to go. Go down. The upper levels will fill with smoke and the path up will probably not be safe.

Go down via the closest staircase possible.

All the gases will go up, fresh air will be sucked in from the bottom and the smoke and carbon monoxide and dioxide and everything else will be pushed upwards and out that top exit. This will fuel the fire at the top initially, and make it stronger.

Last person out of a room / level shuts the door but doesnt lock it. This can help limit the fires growth (by cutting off fresh oxygen) and give other people more time to get lower.

Firefighting will be a pain, as getting access to a room will introduce oxygen to a hot room with imperfect combustion, this will result in flash over.

You'll have bottlenecks in your stairwells. A well designed structure will have fire doors around a large landing adjacent to the stairwells, and wide stairwells suitable to get the population downstairs quickly.

  • $\begingroup$ "When in doubt, Bottom Out!". Words to live by. $\endgroup$
    – Willk
    Commented Sep 19, 2020 at 19:38

As long as the byproduct of the fire are hot, they will try to move upwards, unless something prevents this motion. This include smoke and carbon dioxide.

That's why the general advice when leaving a burning building or plane is to crawl on the floor: there the air is slightly more breathable.

Since in your case the fire has broken in the top area and the top area also has a large open volume, the most logical evacuation route seems to be again the bottom exit.


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