I understood the term "dungeon" in this thread to mean the typical underground environment as used in D&D adventures: extended underground complexes, inhabited by an admixture of different creatures, and being their own type of societal system.
I generally accept the term "dungeon" to mean a wider range of environments than that. For example, "Castle Ravenloft" is a castle, but also a "dungeon". The Pyramid in "Pyramid" is well, a Pyramid shaped tomb/temple complex, but also a "dungeon". As for inhabited by a mixture of different creatures and being their own type of societal system, in the real world we really only have humanity (plus perhaps its pets and vermin) as possible example in that role, so naturally we won't have the Star Wars cantina diversity possible in a D&D setting. Nonetheless, for these purposes I'll largely confine my examples of real world (potential) Dungeon complexes to underground complexes, and not make much mention of examples of above ground abandoned ruins of the real world that would make or inspire excellent "dungeons" in a monster filled world - say Alcatraz Prison - even if it is a literal dungeon.
Out of curiosity, what real-world underground dungeons do you refer to? On that scale, I can only think of the underground cities of Cappadocia, maby the catacombs of Rome.
Actually, the catacombs of Rome aren't that extensive. They are more numerous than extensive, as there are like seven or so smaller complexes. The real champion for real world underground dungeon of that sort is the Paris Underground, whose catacombs, subway tunnels, sewers and the like extend for literally hundreds of miles of passageways. Like many truly emmense real world dungeons, these began as stone quarries. People needed stone for the city above, and it was convenient to mine it from underneath wherever you were going to put the building and just raise it to the surface. Gradually, these got larger and larger. Eventually people decided that it would be a waste to not use all that space, and converted them over to things like catacombs - necessary because the graveyards of Paris were in some cases raised to 20' high because of the bones accumulated there.
While smaller, the London Underground, Moscow Underground, and New York underground are or similar scale. Not surprisingly, many of the largest and most famed dungeons of my homebrew world have exactly the same sort of backstory as the Paris Underground - people built where stone could be quarried, the town grew, and they just kept digging up stone.
Eventually they started repurposing these spaces as catacombs, temples, fortresses, viaducts, cisterns and sewers, marginalized people started treating them as homes, and in the abandoned parts things moved in. If you want a truly collosal dungeon, just imagine one of these huge cities and its underground gets abandoned or taken over by monsters. That's more dungeon than you'll ever be able to use.
Other sorts of mines get similar vast and complicated. The Wieliczka salt mine in Poland has 180 miles of passages, and more or less contains an underground city for the workers (and again, repurposed space). And there are literally dozens of massive mine/quarry complexes of that scale around the world, many of which date back to antiquity, they just don't all pop into my head right now.
Another common real world dungeon is the underground fortress. Cheyenne Mountain and Burlington fall into this category. China has several of that scale, including one under Beijing. Washington by rumor is honeycombed with these sort of things, beginning with an escape route beneath the White House (which has at least six sub-basements) designed to get Lincoln out if the city fell in the Civil War, and just expanding thereafter. Many of these military 'dungeons' have literally hundreds of miles of passage ways, and underground cities meant to house thousands. The Maginot line though is probably the champion in scale however, as it was meant to house literally 100,000's of thousands of troops. On the smaller scale, there are literally hundreds of abandoned missile silo complexes that are as large as typical "dungeons" in stand alone modules.
You mention Cappadocia, which is merely the largest/oldest of many underground city examples. For your typical 50 room dungeon, there are many more examples.
What you don't mention is all the natural cave complexes in the real world, the biggest of which is Mammoth in Kentucky, but there are hundreds if not thousands containing 10 or more miles of passage around the world, and uncountable numbers as large as the average dungeon. Lechguila/Carlsbad in New Mexico, Wind and Jewel in South Dakota, Ox Bel Ha and Nohoch Nah Chich in Mexico, Er Wang Dong in China, Holloch in Switzerland, and Optymistychna in the Ukraine have a scale that puts most anything that has been published to shame. This is real world on a scale that approaches the 'Under Dark'. String a few together and populate them with beasties, and you'd have more Under Dark than you could explore in a campaign.
And again, this is just "underground", and I'm only focusing on 'megadungeons". Once we expand into ruins we get literally hundreds of amazing ruins of every sort we'd find in D&D module, many of which are bigger than most D&D modules. The Pabst brewery in Wisconsin would make an amazing dungeon.
Considering the other list of idiosynchrasies, they really refer to the classical, one level on a sheet of graph paper dungeons of old. As an example, let me quote from memory some "features of the underground lair of the naga in N1: Against the Cult of the Reptile God...
N1 would be a classic example of a low realism dungeon. And, at best, I confess that looking at an 'old school' dungeon like that I have the sort of condescending appreciation for it that an adult might have for the work of a child: "Oh isn't that nice. And you stayed inside the lines too." Given that it was a first attempt in a newly discovered artform, allowances must be made, but the entire thing looks so sloppy, amateurish, and ill-conceived to me now that I'm just entirely boggled. As a 12 year old DM in 1985, dungeon features like that bothered me then, and been beneath the standards I held myself too designing my own. (The map in X1: Isle of Dread has similar hydrology issues, and had to be redesigned.) And that was before I took college classes in speology.
All in all, this makes no sense to me. But it's a fun dungeon, anyway.
And that's the gist of it: a dungeon doesn't have to be logical or scientifically correct.
If someone was running the dungeon for me, I might enjoy myself, but I'd be also thinking, "Nice for a first effort, but isn't this corridor supposed to be running through the middle of the lake we boated across on level 1? And for that matter, isn't this entire complex below the water table?" If I purchased the module today, I'd be somewhere between angry and resigned: "Oh great, another module where I have to completely redraw the map! Doesn't anyone have a sense of craftsmanship these days? I thought we got past this crap in the mid-80's!"
The point being, there is really no excuse for having a dungeon that is nonsensical but lack of skill and knowledge. The real world doesn't constrain your imagination - it informs and expands it. There is no classic dungeon that you can't point to examples that would be of the type in the real world; all you have to do is move the monsters in.
