It is of frequent comment that if D&D had any sort of verisimilitude, there would be no dungeons. (Less commented: no dragons, either.) And if we’re talking about about endless warrens of perfectly dressed stone corridors, an incredible megainvestment left fallow for occupation by grubby little goblins shitting in their middens, those commenters are probably right.
Imagine the cost of something like building a Krak des Chevaliers, only you have to dig out all the space before you can even begin to build, and then backfill over it. Or tunnel every bit of it out, then maneuver all the materials inside. And then . . . abandon it? Invade it and extirpate the inhabitants and not make it the fortress center of your own empire?
Silly, sure. But we need the D in our D&D. (Did you really mean to say that, hella? Probably not.)
So we need other reasons to have massive underground complexes with all sorts of cool shit inside, rather than shivering half-starved refugees and the occasional eyeless lizard. What can we come up with?
Mines. We need a remunerative reason to go to the effort of scrabbling thousands of cubic yards of earth out of the ground. Defensive structures, at least at scale, isn’t enough: it costs about fourteen billionty times less to build structures atop the earth, and the defensive benefits of digging down just aren’t that great. Unless you’re defending against pitiless airborne firebeasts—and it takes a lot of dragons swirling the skies to makes it worthwhile to just not try and hide—a real fortress is almost as good and many times easier.
But there has always been at least one amazing reason to dig tunnels, and that’s where the digging is the purpose; the galleries and passages left behind, an afterthought. If the substrate is strong enough, or enough care put into shoring everything up, you’ve got a system of adits, drifts, and shafts that will last long after the ore is gone. In a dangerous world, a securable structure impermeable to the elements will always find itself reused.
Caverns. Yup, you already thought of this one, and Patrick Stuart already did it better. The key to it not being stupid is to think in three dimensions and remember that water carves out shapes that don’t give a shit about habitability.
Something else to remember is the weight of history in any easily accessible rockshelter. Something so easily found and used, out of the weather, will have been continuously inhabited—since time immemorial. Think middens forty feet deep. The psychic weight of thousands of generations feeding, fucking, and fighting all in this space. You ain’t the first adventurers burning this place down, and you won’t be the last.
Qanat. Over the longest term, what’s underground and worth more than ore? Water. And how do you get it from the depths onto the crops? Tunnels. You find where the water table rises in the hills. Tunnel sideways into that water table, and let gravity pull the water out along those tunnels to where it emerges into the plains. Shafts down allow you to pull water up along the way, cisterns hold water for when it’s needed, and side-canals widen the area watered.
When the water is flowing, such an underground complex is the single most valuable thing in the area. When the water dries up, then someone else moves in.
Paleoburrows. People didn’t make this. Megafauna did. Whether giant ground sloths clawed them out, or some even more fantastic beast clawed, chewed, or swam through the earth to make the tunnels—some enormous beast burrowed into the earth, making tunnels and chambers that remain long after the original inhabitants have gone.
Or, maybe they haven’t really gone. If that thing clawed through hundreds of meters of stone, you think your hauberk is going to stop it?
Rootspace. Something enormous used to draw its nutrients from the earth. Vast roots sunk deep, supporting an equally enormous plant above. Or maybe enormous rhizomes, the undergirding of innumerable shoots aboveground, lay hidden and hulking below. But it died, and rotted, and scavengers and water and time flushed out the organic material, leaving voids below the earth.