Another idea is that the aberrations in the dungeon made a deal with the inspired. Not make trouble and "take care" of delinquents and being left alone in turn.

Both sides being what they are, a conflict would just not be worth it for either side.

Quote from: Shipyard Locked;784259The questions are "Why hasn't the very organized and powerful dictatorship already cleaned it up?" and "What's to stop the dictatorship from cleaning it up themselves the moment their competent spies notice what the PCs are doing?"

Perhaps the dictatorship rules at the pleasure of those below. So long as the government feeds the dungeon fresh victims its masters allow them to rule the surface.

If too many "victims" make undo amounts of trouble for those below then the powers that be will put pressure on the surface government to take care of the problem....

The PCs if they are too successful in the dungeons below, could find themselves hunted by both the dark masters below AND the dictator of the city above.

Maybe it's too much trouble to explore the dungeon.

Are they causing any problems outside? If not, then it is effectively a ghetto, with all the inhabitants trapped inside.

Perhaps there are useful people in there, they can't be allowed to wander around outside, but could provide some services.

If the dungeon is vast, perhaps the authorities don't appreciate how large and complex it is. This might be true where several complexes have been merged into one. Perhaps a set of catacombs, a set of mines, old sewers, part of the city ruined by an earthquake and built over, fugitives burrowing into the hill and natural caverns. Join them up with a set of tunnels and you now have a massive complex where before there were a few odds and ends.

5 minutes of brainstormed reasons why access to and/or knowledge of the megadungeon is limited:

- The MD has a controlling intellect a la Undermountain who is selective about who comes in (and out!) of his/hers/its creation.

- Ancient wards that do a "soul check" at access points; only 3rd level or higher sentient beings are allowed in.

- Anyone can come in; a sacrifice of gold or gems or a specific key is required to exit.

- It's a mythic underworld: http://muleabides.wordpress.com/2013/09/16/logic-and-the-mythic-underworld/

- The MD is an extrusion of Hell into the Prime (or whatever terms fit your cosmology, I'm only passably familiar with Eberron).  The laws of physics and metaphysics don't necessarily apply (see Queen of the Demonweb Pits for an early take on extraplanar rules alterations).

- A melange of some or all of the above- each level or sublevel has it's own raison d'etre;  PCs not only have to map the dungeon but keep track of how each portion deviates from the world above.

You may also look at the Crepitus Caves in Jagged Alliance 2. Over there, the draconic dictatorship has actually been the one to bioengineer the monsters in the first place, and keeps the crepitus plague docile by throwing the bodies of the dead, the dissidents and the traitors down the bug hole. Things turn ugly when the feeding stops and the menace crawls to the surface looking for food...

Crib from Stonehell: the dungeon is a mad experiment being run by a major member of the government, or at least started that way.

Quote from: Shipyard Locked;784214I still don't like it much; it makes the ominous empire look weak.

There's your rationale.  The ominous empire can't deal with it effectively (either it's too big, or too well-defended, or something) and knows that, and can't afford to let that become widely known lest the empire appear weak in the eyes of its enemies.  Further, different factions within the empire disagree on what to do about it, from clearing it out whatever the cost, to using it as a tool, to walling it off and forgetting about it, and this internal dissension is preventing anything from being done.

So the empire itself will act to conceal anything the PCs do down there and the PCs now have factions within the empire they can exploit.

I'm thinking the Balrog(etc) is at the bottom.  The empire knows this, but discovered it well after the city was considered a success.  They know they can't relocate the whole city.  They also cannot possibly hope to kill the thing, and the last time they tried - well let's leave it at 'it was bad'. They also know the 'Thing Down There' needs to eat someone, and allowing a 'buffer of scum' to build up actually protects the citizens - so long as all the bad stays down below.

Sort of a Cabin in the Woods deal...

It introduces a thin veneer where you get to decide who is in the know and who is not.  I'm picturing a middle commander who wants the place cleansed but his superiors keep blocking him below a certain depth.

Thanks everyone!