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[Dorsidwarf]

Very interessante. A "few dozen 1-2 tile openings" is a lot me thinks. That's hundreds of meters worth of space--and. At the very least, on a grand scale, I imagine pressure would indeed dictate that oxygen makes its way into the cave systems in considerable quantities. On the flip hand, I think it's probably impossible for an average sized DF world to support an internal vacuum of the size of the cave systems without imploding.

*also size/scale is somewhat problematic. Tiles are TECHNICALLY about >2m^2, but also you can fit multiple sperm whales in a single tile so... I imagine that the entrances into the underground are at least large enough to fit two adult sperm whales. The average width of a sperm whale is 20.5 meters and since you can indeed fit more than one in a single tile space,  you COULD say that the openings into the caves are roughly 41m^2. So let's say there are 36 such openings at 2 tiles a pop in a world, that's almost 3000m^2 of capacity.

It may be a lot of space compared to most real caves, giant Vietnamese aircraft hangar ones aside - but remember that the cavern system is thousands to millions of times bigger than any cave network in the world, seeing as it underlies literally the entire world. Anywhere in a DF world that you go, you just dig down a few dozen meters and you’ll find them, either the claustrophobic passages or huge cavernous spaces with lakes and mushroom forests. Mammoth is the largest real world “caverns” and it has a grand total of 400 miles of passage. Imagine Mammoth cave network but it’s so expansive you can reach absolutely any point in the world by taking an easy hike through it, and that’s what the DF caverns are

[Tubercular Ox]

It may be a lot of space compared to most real caves, giant Vietnamese aircraft hangar ones aside - but remember that the cavern system is thousands to millions of times bigger than any cave network in the world, seeing as it underlies literally the entire world. Anywhere in a DF world that you go, you just dig down a few dozen meters and you’ll find them, either the claustrophobic passages or huge cavernous spaces with lakes and mushroom forests. Mammoth is the largest real world “caverns” and it has a grand total of 400 miles of passage. Imagine Mammoth cave network but it’s so expansive you can reach absolutely any point in the world by taking an easy hike through it, and that’s what the DF caverns are

For this reason alone I choose to believe in cracks and fissures, independent of what else that can justify.  As soon as Toady implements weather simulations, the caves we can see become barometric singularities, unless there are additional openings leading to the caverns.

Also, taking scale strictly as is leads to magma often being 150m below the surface or so.  Given traditional insulation values for solid rock (and ignoring the convecting power of the cavern layers), that would make the surface hot enough to boil water.  That's a neat trick when it's a lonely volcanic island, not so spiffy when it's the entire ocean.

Personally I treat scale for open space strictly but imagine solid stone to be more flexible in what it portrays.  I've got the first layer of caverns pegged as just under the aquifer, possibly at a to-scale depth, the third layer at around 1km under the aquifer, because that's about as low as a carbon ecosystem can go before ROI turns to dust, and the magma sea way down farther than that.  But I can only argue for those depths in a "realistic" world that looks like dwarf fortress wherever possible but isn't bound to it in every particular.

And since I'm already taking liberty with the principles of the world, sometimes I wonder if literally the entire world has caverns.  The largest possible world map, following strict ground scale, is around 150,000 square kilometers.  That's smaller than Florida, Florida is soaked in karst (terrain good for caves) https://people.uwec.edu/jolhm/Cave2006/Karst.html but look at all the parts of the US without karst.  What if caverns only look like they're everywhere because dwarves are mostly endemic to them?  What if finding a piece of the world without caverns is as easy as going into the advanced worldgen settings?

[Urist McScoopbeard]

Ah, but is it really the natural DF world if we use advanced world gens???

[Tubercular Ox]

Ah, but is it really the natural DF world if we use advanced world gens???

How would we know?  Seriously.

[Man of Paper]

What if we are the advanced world gen?

[TomiTapio]

Sulfate-breathing microbes:

"Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42–) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S).[1][2] Therefore, these sulfidogenic microrganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration."

[Tubercular Ox]

Sulfate-breathing microbes:

"Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42–) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S).[1][2] Therefore, these sulfidogenic microrganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration."

That's one of the anaerobes I was referring to.  There are also methanogens, who use hydrogen gas, and denitrifying bacteria, who use nitrates.  The SRBs form a loop with the unfortunately acronym'd Sulfur Oxidizing Bacteria, as the other two form loops with methanotrophs and nitrifying bacteria.  And then there's the rest: https://en.wikipedia.org/wiki/Anaerobic_respiration#Examples_of_respiration

My favorite are the nitrifying/denitrifying, because they have the vast amounts of nitrogen in the air to feed into the loop, but the sulfur set give a better impression of being able to go it alone, especially where cave gases are involved.

Those loops do make it seem like things could go on forever without energy input.  There's a surprising amount of energy extractable from a simple cline, thermo-, oxy-, or otherwise, but entropy does get you eventually without a source of new energy (normally the sun, but potentially core heat as converted by adamantine).