Mythic Underworld but not quite
I never really got into the idea of Mythic Underworld. I always thought of dungeons in terms of ecological reality, even if warped by magic. Problem definitely came from conceptualising the whole premise of Mythic Underworld. But there is one medium that helped me quite well in understanding it as well as kinda twisting it to my own liking.
So the whole idea of Mythic Underworld is that dungeons explored by characters in OSR-adjacent games are not the regular reality, these are places of wonder, of impossiblity and also of hostility - as the old rules for doors constantly locking behind characters, impossibility of monsters living underground and so on. The idea that dungeon itself is the enemy and works actively against characters because it is some kind of mythical place.
I don't really like it like that. As it sometimes is with exploring Wilderness it otherises the non-civilised, non-"our" parts of the world. As if the humans were some stabilizing force in the world and once we're somewhere the wonder fades. And it is a kind of fantasy but it definitely not type of fantasy I do want to use in my games. It puts emphasis on human vs other but it doesn't really follow through, it is not really addressed besides vague hints. I would rather have it explicit - you're humans and you fight against world of Magic/Chaos/Paradox or you don't.
But there is a video game that makes it kind of organic in a sense. This game is Fear and Hunger. I'm not really a fan of combat mechanics of the game - I simply can't stand JRPGesque turn based combat - but aside of that the game is about exploration and interaction. You start at the entrance to the dungeon in which prisoners were kept and everything seems weird and dangerous but enemies are still human and animal, rather mundane. Only after you get deeper in the dungeon, in the mines and below you encounter more weird, unnatural things. When you actively get closer to mystical powers of the world you meet with more mystical creatures and powers.
For me this is more compelling thing. Not that Underworld or Wilderness is by itself Mythic, but by influence of something beyond. You can go through the wildest woods in the world and encounter only wildlife. But if Fairy Court uses it to travel through, Eldritch God marked it with their glyph or something only then it becomes wondrous. But it may also be mundane place, even if a dragon made its lair nearby, it doesn't require the Mythic to come here. It should have, in my head, specific source, and not like general rule of the world.
And now it feels compelling to me. Not every goblin cave is special. Some are just cave full of goblins you fight and then move on. But some have secret passages to civilizations under the mountain, some have cracks that smell sulphur and lead to hell and others change the way out, so when you leave the cave the world around is a different place than when you walked in. And some don't activate until you do specific thing, so these may be mundane places for you but not for other folks. Shit, elves and dwarves can work like this.
Like, you can wander to a dwarven mine by mistake but if you didn't go through main gate you just find a solid straight tunnel below a mountain range. Where are all these dwarves? But if you go with dwarven blessing you find fantastic chambers, dwarven communities, wall carvings of epic scenes, gems, magical lanterns, orcs and saurians trying to breach from below and dragons trying to get inside and burn these pesky jewel hoarders.
I can also see this as if Mythic Underworld was a rule of the world and if you dig deep enough in mundane world you would eventually uncover dwarves, orcs or magic. And only by robbing and killing them you'd turn it all to soil again.