What's beneath a prison?

I'm working on an adventure taking place in a prison; the players have committed some crimes to end up here and will need to plan some way to get out. I'm planning on having an 'earned time' system as some modern prisons do wherein the players can do work to reduce their sentences. In my story, this means working with the guards to reclaim some of the lower floors which have become monster-infested. However, there's only so many times I can have the players clear out some old cells before the size of the prison starts to be ridiculous and they get tired of the same scenery.

What might be some fun ideas for these lower-level rooms (e.g. apart from more cells)?

For example, the prison could built above an old mine-shaft, so there'd be various rooms relating to the mining process, e.g. acid-baths for softening the soil, derelict mining machines, etc. Pushing this into the fantasy world more, the drilling machines might be built on beasts and there could be temples/labs dedicated to controlling/praising them. Some other ideas I've been kicking around are a secret treasure-room housing goodies confiscated from prisoners, a moonshine-brewery, interrogation rooms, an underground casino, and a graveyard for accidental deaths. 

Temple (probably ruined, maybe connected to the graveyard?)

Wizard's laboratory (where the wizard possibly experimented on prisoners to turn them into monsters)

In a D&D world?  Probably undead, eldritch horrors, or the Underdark.  Possibly undead eldritch horrors from the Underdark.

The source of the Faerzress? Lolth's nursery? Jergal's resurrection seed? A nothic colony? Tzeentch's (erotic) library? Shar's gauntlet (if you don't mind being derivative)... Angband for good measure... A rakshasa's dream paradise? The not-quite-dead body of an Empyrean? 

Maybe a prisoner long ago was such a horribly evil wizard or warlock or whatever that they were locked in a cell down deep in the sub-bowels of the prison. And with nothing but unlimited time and privacy on their hands, they performed some evil ritual that opened up a planar gateway between that cell and Carceri (aka Tartarus), the outer plane that is quite literally one giant prison. 

So there's that. 

Unless you have a reasonable explanation of how the monsters got there, have had adequate food and water all this time, way of disposing of their waste, ways to reproduce without over populating... you are better off using undead, vermin, fiends, etc. Perhaps the prison was built over the site of some ancient undying creatures that have been locked away, and now that the prison has excavated an opening - want to get out.

I suppose you could also use a path to the underdark, but if the area below the prison contains intelligent beings, what will the response be by those even farther below when those stop reporting back? When it is discovered that they are dead, what will those farther down do about it. Unless you plan to have the PCs (or someone else) seal the area back up again, it could open a can of worms that could end up overrunning the prison (and more).

Perhaps just have them work in a mine (facing hazards and vermin, instead of intelligent monsters).