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Message: <blockquote data-quote="Celebrim" data-source="post: 5644393" data-attributes="member: 4937">I'm not familiar with either the catacombs of Malta or Rome, but I have a fair amount of knowledge of the Parisian ones and, at least in Paris, air shafts are a natural result of the most expedient quarrying technique.  Typically, rather than extending the shaft horizonatally and then have to manuever the stone through the passages and up some human access point (which would have stairs and the like) and then back to the construction site, they'd drop a new vertical shaft near where the stone was needed.  Then working from the base of the shaft they excavate new stone and lift the stone straight up through the vertical shaft using a simple crane.  The result is that the quaries would have air shafts at many points.  However, I know that there are lengthy underground stone quarries in Russia that used completely different techniques.  From what I can tell, these where so large (hundreds of kilometers ultimately) that they ended up working like a natural cave system.Hypothetically, a living cave which has a stream containing large amounts of disolved oxygen could release the oxygen into the cave system thereby replacing the need for an air opening.  This could explain the presence of 'good air' on the other side of a sump, though in most cases I imagine that its really the cracks involved.  However, I know that the opposite impact of a stream is certainly possible - oxygen poor water entering the cave air disolves oxygen out of the air faster than ventilation can replenish it.  This is one of the three or so more important ways that you get bad air in caves.  The third is having mineral deposites in the cave that rapidly oxide - iron rich much, walls rich in sulphur deposites.  Deposition of oxygen on the cave walls can occur faster than natural ventilation replinishes it.Open flames on the other hand are very good oxygen depletion detectors.  Characters carrying torches would be warned quickly of bad air by observing that thier torches are burning unnaturally low.  Aerosole tests like a cigerrette lighter are even more effective, and can be used to actually measure the amount of oxygen in the air, but I doubt such devices are common in most campaign worlds.Relying entirely on artificial light - such as a magical flame or light source - is concievably even more dangerous because you'll get no warning until your Wisdom has already taken a big hit because your brain is starving for air.This assumes that access to high level magic is something that is readily available.  I suspect that for most campaigns, ordinary solutions are more dependable and easier to come by.</blockquote>

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