[This was originally posted as a comment on a Reddit post but, knowing that will disappear down the memory within a few hours, figured it was worth preserving here as well]
In Original D&D (1974) the advice and procedures for creating dungeons were very strange: dungeons were supposed to be infinitely large and ever-changing, filled with a mostly-random assortment of monsters, treasures, tricks and traps with no particular reason or justification, and there were even weirder rules like that doors are always stuck for adventurers but never for dungeon inhabitants, and that all dungeon inhabitants can see in the dark unless they’re in the service of a PC in which case they lose that ability.
By the time of AD&D and TSR's first published modules (in 1978) Gary Gygax had mostly moved away from that mode of design and towards a more logical and rational style that James Maliszewski later dubbed “Gygaxian naturalism” (though that is something of a misnomer since other folks/games like RuneQuest and Chivalry & Sorcery both went there first and leaned it to it more heavily - Gary always kept one foot in each camp) and the earlier mode was derided as “funhouse” style and looked down upon, and was largely abandoned by the early-mid 80s (The Abduction of Good King Despot, published in 1987, was probably the last gasp of this style of adventure in the Classic Era). Which was a shame, because that kind of game can be a lot of fun, especially compared to overly-ecologized stuff which can be dry and boring (especially when it jumps through so many hoops to explain and justify its “fantastic” elements that it drains the thrill and wonder from them).
Fast forward about 20 years to the early 21st century, and a few of us were trying on forums to revive the legacy of that old style, to bring back more of the sense of freewheeling fun and adventure that we felt had gotten lost and buried in 2E and 3E D&D. So we went back to the earliest material (books, fanzines, and testimonies of first-generation players) and advocated for the way they did it then and that the game could still be played that way and would be as much or more fun than the other approach. But that effort was hampered because people kept harping on the lack of logic and realism - declaring that everything was arbitrary and dumb and simplistic and they couldn’t suspend their disbelief enough to enjoy playing in such an environment.
Frustrated at being put on the defensive and having the same arguments over and over again, a couple of us decided it was worthwhile to come up with a rhetorical justification that went beyond the reductive “it’s just a game lol” excuses. I was reading both Lovecraft’s Dreamlands cycle and Joseph Campbell's Hero With a Thousand Faces at the time and realized that all of the weird elements of funhouse-style D&D made sense in a world of dream-logic and in the context of the mythological hero's journey; combined with the notion already present in D&D lore that almost all of the classic “funhouse dungeons” were built and overseen by a hostile or insane demiurge of divine or near-divine stature (Zagyg, Halaster, Zenopus, Keraptis, Acererack, Ignax the 27th, etc.) and it all kind of came together. I was (I think, but don’t have any receipts to back it up) the first person to articulate the idea of the dungeon as a mythical otherworld that is specifically counter to the normal logic and natural laws that govern not just our world but even the mundane parts of the fantasy world (the towns, wilderness, and “lair-dungeons” that operate on (what would later be called) Gygaxian Naturalist principles); that entering the dungeon is literally crossing the Campbellian Threshold to Adventure into a mythic otherworld. Either way, a friend of mine named Jason Cone (who goes by the forum-handle Philotomy Jurament) took this idea and ran with it, expanding and formalizing it into an essay that he posted on his blog c. 2005ish.
A few years later, following Gygax’s death, the controversial sacred-cow-slaughtering shift to D&D 4E, and the release of OGL “retroclone” games like Labyrinth Lord, there was a sudden upswell of interest in the oldest forms of D&D with tons of blogs (Maliszewski's Grognardia chief among them) popping up on the subject and bringing those old forum discussions to a new and wider audience. These guys were all really taken with Philotomy’s essay on the Dungeon As A Mythic Underworld and it became something of a foundational text to the budding OSR movement, alongside Matt Finch’s Quick Primer for Old School Gaming (which popularized the "rulings, not rules" concept that also grew out of those same forum discussions). Between those two essays, all of the things about 70s-era D&D that had been so roundly dismissed as dumb and primitive and broken in the 80s and 90s now had a sufficient rhetorical and game-philosophical justification that people felt freed up to play that in that "old-school" way and have fun with it without having to feel guilty or defensive about it.
But now, another dozen-plus years later, that’s all ancient history. The maxims of Rulings Not Rules and Dungeon As Mythic Underworld have become OSR dogma, stripped of their original context and purpose - i.e. to oppose the then-dominant contrary trends and justify a style of play that had been denigrated and dismissed for decades. What got lost is that these concepts weren’t posited as the only or best way to play, but as an alternative. We never intended to claim that rules are always bad, or that all dungeons should be mythic underworlds and normal logic and ecology should never be employed. On the contrary, one of the original points of the dungeon as mythic underworld is that it’s an exception to the fantastic-naturalistic logic and ecology that govern the rest of the game-world. To me there’s an ideal balance between rules and rulings, between logic and symbolism, between reality and dreams, that Gygax, Jaquays, Stafford, Perrin and a few others instinctively hit c. 1978-82, that really thrills and inspires me even to this day. The pendulum swung too far in one direction in the 80s-90s and diminished that magic, then our attempted correction in the 00s caused it to swing too far in the other direction with the OSR in the 2010s.
So if you feel [like the Reddit poster this was written in response to] that the Mythic Underworld concept is overused and is too often used as an excuse for lazy or sloppy design, I say that you’re right, and do so as one of the first people to articulate the concept and inspire the guy who popularized it. There’s a place for dream-logic non-naturalistic dungeons, but they should be (at least in my opinion) a fairly minor ingredient in an otherwise Gygaxian Naturalist stew rather than the only ingredient in the pantry.