But then I stormed a brain and realized — orcs on Rockulon Prime were originally vat-bred by the drow, like Peter Jackson's take on the Uruk-hai. Being that the drow were sufficiently advanced, it makes perfect sense that birthing pits would somehow be self-regulating (or at least some of them would have been). Probably some mild artificial intelligence, making the whole thing like a sentient ooze that generates orcs.
So what if one went bad, like cancer? A cancerous flesh labyrinth, burrowing deeper into the earth, spreading corruption as it goes. A malign, insane, genius-level intellect guiding the dungeon.
It would still have traps and monsters, but these would be part of its immune system. There would probably also be mutant orcs as its birthing pit protocol became increasingly corrupted.
It could probably also have treasure — it produces bezoars and pearls and weird mineral deposits that work like gems, and it has probably absorbed precious metals and magic items. It probably has caches from adventurers who previously attempted to delve it. Or it created weird, organic devices of its own design, looking like chaositech or Yuuzhan Vong nonsense.
For that matter, what if Quimmermount needs organic material to generate further legions of progeny? What if it is deliberately attracting adventurers to generate the necessary material, supplementing it with whatever its pseudopods can catch.
So, Quimmermount. A cancerous orc birthing pit worming its way into the earth. A living, breathing megadungeon. It's a thing.
Brainstorm the Second: By the way, if you run a game like Lamentations of the Flame Princess which tones down the fantasy elements and makes creatures unique, perhaps this is the only source of orcs in your campaign setting. Maybe orcs are relatively rare, only haunting one particular location with Quimmermount at its heart.
So when some baron asks you to look for these greenish, pig-faced monsters haunting, say, the Black Forest, you find the corrupted birthing pit at its heart, no doubt made by some antediluvian civilization for unknown purposes. Or maybe the original birthing pit seed fell to Earth in a meteor strike, and has only now grown enough to start spewing out its bio-engineered progeny.
Maybe orcs are humanoid because it uses human DNA as the template.
I'm just rambling now, but there you go.