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Message: <blockquote data-quote="Celebrim" data-source="post: 5644061" data-attributes="member: 4937">I'll give it a shot.Rome has many catacombs, but each are relatively small.  Ventilation is probably provided by the expediency of having multiple entrances over a comparitively small area.Carlsbad is huge for one thing.  In general, cavern complexes are ventilated by 'breathing', in that a cavern complex generally maintains a fairly constant air temperature.  When the outside air is warmer than the interior air, air blows out of the cave complex.  When the outside air is colder than the interior air, air blows into the cave complex.  This cycle often occurs daily in many caves.  Carlsbad's ventilation problems are more complex than most caves though because Carlsbad naturally vents sulferic acid fumes.  I'm no expert on Carlsbad, but I would suspect that there can be pockets of bad air in back areas of the complex where visitors would not normally go.  However, given the massive size of the Carlsbad entrance I would suspect that there is alot of circulation going on, and the breathing problem is made easier by the fact that the fumes - being lighter than air - rise rather than sink.Caves normally only have problems with 'bad air' if they contain so much life (and associated decay) relative to the amount of air flow, that carbon dioxide or methane pile up.  Insects and other animals can be adapted to the oxygen poor environment, while it still being unhealthy and potentially lethal to humans.  This is true of, for example, many caves in Mexico.  It's also true of some caves with large bat colonies.I was not even aware of the existance of this real world 'moria', and will have to give you XP for bringing it to my attention.  So far as I can tell, the inhabitants of Derinkuyu built an elaborate system of ventilation shafts to ensure there would be air circulation throughout the complex.</blockquote>

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