RIght, so, this is a bit of a throwback question.

If your setting has dungeons that are structured and stable -- with traps and treasures and most importantly can be re-visited or explored over time...

Who runs them?

Who builds them? Who rests the traps and the tricks, who designs the intricate puzzles and all that stuff?

For my previous campaign setting, it was a Demon Lord (Lass, Lady? they are a complex being) who built it on a nexus of dimensional space where a tar in the warp and weft of reality had opened. The campaign was built around the dungeon and the underlying story around it, so it was easy.

Previously, I had used Kobolds as the engineers of such things.

For my next campaign, a lot of it is done by a race of people called Imps (the standard grey skinned, winged, tool wieling sort, but not infernal or demonic or devilish, just people). Ips also created the UnderDark in that setting, and it is their home -- although most of the lore that goes to players is not inclusive of that fact. It is thousands of miles in size, mostly about a mile underground, and so they have all manner of ways to ventilate it, and they can survive the punishing environment (heat, gases, moisture, etc).

THe "dungeons" that might be encountered are almost inevitably going to be connected in some way to the Underdark, because the Imps got there first. Ruins (which are frequent), old wizard towers ((who might have made a deal) and similar stuff are all there, and many of the risks are from IMps -- who also have the ability to enter any city or town and steal or rob or otherwise relieve them of the burdens of their fancy stuff, which ends up placed into those locations as storage grounds (which is why it is always neatly stored in chests and such).

THere are actual Dungeons, but I was told I had to limit myself to five, and so they are scattered about in out of the way nooks and crannies, and may or may not have imp involvement (but all have planar involvement).

just lightly touching on the idea, but I was curious about others who have actual dungeons and such -- what's your deal?

I have to say I like the idea here of, basically, the dungeon keepers union, never thought of that before. 
Typically, it’s just whoever lives there for me, as I think a dungeon is more a series of encounters around a single theme in a single place. A cave complex can be a dungeon. A really nice house can be a dungeon, etc.

Quote from Xalthu >>

I have to say I like the idea here of, basically, the dungeon keepers union, never thought of that before. 
Typically, it’s just whoever lives there for me, as I think a dungeon is more a series of encounters around a single theme in a single place. A cave complex can be a dungeon. A really nice house can be a dungeon, etc.

They can indeed, and I have often done that. But I also have a LOT of side quests and odds and ends floating about, because you can just never tell what players will do, so I have ruins and abandoned places, where no one lives, that get raided or rumored and stuff like that.

I have an entire culture for the Imps, distinct from the other peoples, and it revolves very much around the core idea of people whose job it once was to provide the logistics for an army.

The mix of the two -- Imp run places, inhabitant run, partnerships -- all gives me the ability to rapidly have a 'backstory" for most things, 

Interesting thought - I hadn’t really ever considered that but in one sense I have (at least) 2-3 such dungeons in my version of FR - other than the published 2: Larloch’s crypt and undermountain. There are two that are what I would call semi- published. The first is deep under the Anauroch city of Oreme where there is a Sarrukh city run by the Terraseer and 4 other great Sarrukh Lichs taking 5 year turns. WOtC published it - in part- as part of a 3.5 adventure leading up to the spell plague. The other is similar - a buried crashed Netherese city ruled by a Netherese arch Wizard turned elder brain. Both have had some description published but neither is detailed like undermountain (nor is Larloch’s crypt). But given the history of the realms there could be quite a few similar places including the nameless dungeon that might be being run either by Fey’ri or ancient elven Balenorns. Other ancient cities from the days of the creator races run by Lichs of one race or another, ancient ( immortal, universal) dragons using old lairs as testing sites or just having gone insane like Halaster and running their own versions of Undermountain. FR lore has identified several possible sites for similar places from the ancient Imaskarian empire. Then there are the nigh infinite layers of deep Calimshan pretty much ruled over by Shoon the IV?. Then there is the treasure dimension of the Thaluds created by the Netherese with a Netherese archmage as the keeper.(another place actually described in 3.5e). If Halaster could create Undermountain then it’s possible that there a number of similar places scattered around Faerun and Toril.

Quote from AEDorsay >>

RIght, so, this is a bit of a throwback question.

If your setting has dungeons that are structured and stable -- with traps and treasures and most importantly can be re-visited or explored over time...

Who runs them?

Who builds them? Who rests the traps and the tricks, who designs the intricate puzzles and all that stuff?

Way back in the day, I came up with a linked set of dungeons that were intended to be a bridge of sorts between the Against the Giants modules and the Underdark modules

They were created and maintained by a powerful archmage who used them to basically conduct social experiments on whatever adventurers blundered into them, viewing them as nothing more than rats in a maze

I tend to think of cursed/corrupted lands more than dungeons, but inspired by The Nightmares Underneath I’ve contemplated dungeons that are created when an area (sometimes even a person) becomes so infected with suffering, malice, and spiritual darkness that it collapses into a liminal realm between the world and some evil plane. Here, the dungeon itself is semi-sentient and can manage itself, or might be ruled by a particularly powerful shadow spirit connected to what cause the dungeon to form.

And since I mentioned that people can become these, it’s also possible to stop the transformation before it happens by entering their dreamworld and cleansing it. By the time it gets bad enough to notice, said dreamworld is probably pretty dungeon-like, and I suppose it’s the person themselves subconsciously (usually) doing the managing. Or a sorrowsworn they inadvertently created, but that still counts as the person on some level.

As a whole the premise of "Dungeons" in the traditional D&D sense is "ancient places".. aka, they aren't maintained, but rather they are just there and have always been there, the characters being the first to explore them since they were built for whatever purpose their original purpose was.

Another tradition above and beyond that is "monsters" taking over the dungeon, aka, a group of Goblins move into an old abandoned underground city for example hence amidst the ancient ruins and "old stuff" that is already there, you have these new occupants altering the location and adding their own traps and stuff like that.

Some examples include B2 Keep on the Borderlands Caves of Chaos for example.

Quote from Fifinger >>

I tend to think of cursed/corrupted lands more than dungeons, but inspired by The Nightmares Underneath I’ve contemplated dungeons that are created when an area (sometimes even a person) becomes so infected with suffering, malice, and spiritual darkness that it collapses into a liminal realm between the world and some evil plane. Here, the dungeon itself is semi-sentient and can manage itself, or might be ruled by a particularly powerful shadow spirit connected to what cause the dungeon to form.

I suddenly have an urge to make a dungeon that's a riff on Danielewski's House of Leaves

I don't do a lot of real dungeons - the sort that are giant predesigned puzzleboxes for the players to solve. But I've done it on occasion, and usually there'd be none of the resetting stuff. Rather, I'd have part of the dungeon already cleared by previous, unsuccesful adventurers, then inhabited by various monsters. Eventually, the PC's would clear the monsters, and find the unfortunate remains of the previous expedition - from which point on the traps would be armed and ready. 

But otherwise my dungeons are more like biomes, or fortresses, or habitats - or whatever. 

For those who may not have ever run a simple campaign wit a dungeon that can be revisited, please forgive me for suggesting that perhaps you should give it a whirl for a few sessions if your group enjoys dungeon crawls.

THe core of my group started out on them -- we get sick of them, and ore often now than before, but we do still enjoy a good one.And when mom and dad are into them and the kids join, they get the same underlying expereince.

SOme of the reasons for this are:

Less time having to add in new maps and create new adventures.

You get a kind of Video game feel -- especially for sections that were done previously.

It prepares players (not characters, but players) for more complex storytelling that isn't Dungeon Based

it allows you to experiment with puzzles more.

For just a few.

Which is not to say they are the best" or anything like that, just that they are an options. And having tem repair themselves adds an extra level of "what's going on". An example of this is my last campaign -- there were a couple of very small dungeons scattered around an area, but there was one in particular that was deep. All of them were there on purpose (the Demon lord, blah blah) but you had to explore them to find where the deep lair was that ultimately led to the UnderDark and the whole nexus thing. It wasn't until 7th level that the characters stumbled upon the deeper levels, and because there are "you gotta eat, you gotta sleep, you gotta haul your gold back, you gotta buy new arrows, you gott..." they couldn't just "stay" in the dungeon, they had to leave and return.

Most of the time they returned, the dungeon was the same -- but slowly but surely they started to note that different parts were changed, or new things appeared, o there was some sign of occupation. 

In my new world, the Imps that are the lords of the Underdark (just people-type monsters, not hellish Imps) spen the years that the main societes spent rebuilding from disasters, wars, and other tings exploring and finding all those ruins of the above ground world. They like that they seem abandoned, tey like that they are the kinds of things that draw people to them, and they like to be creative with their traps and tricks and practical jokes.

But also, those ruins and other places are all secretly entrances to the underdark, which lies a mile below ground. Imps provide the secret ways that enable Goblins to get into the lands and stage their raids and all that, even to the point of creating and holding a base of operations -- usually one that is a ruin that the did not create a linkage to the underdark in, so that the Imps' secrets are protected.

Thus while not all dungeons are managed dungeons, lol, a huge number of them are, and dungeons as a whole serve a purpose now beyond being reminders of some long forgotten civilization/relics of powerful sorcerers/all that remains of the once mighty X of Yz. Granted, I did that so I could justify having an underdark in the setting, lol, but the point still stands.