I see the term “mega-dungeon” pop up a lot. The vast majority of my D&D experience was with 2nd edition, which didn’t use a lot of dungeons (most of my “dungeons” were just castles of various sizes.) When I think of “mega-dungeons” I think of things like Undermountain and Dragon Mountain, but I was led to believe that these were huge even for mega-dungeons.
So what would you consider a mega-dungeon? I’m not talking more about size, rather than play-time. How many rooms/levels/encounters, etc? What old published modules or adventures would you consider mega-dungeons?
Thanks!
The common OSR definition for megadungeon is “big enough for the entire campaign”, ie. even though character’s might do other stuff as well, in a megadungeon campaign the single dungeon is where they spend most of their time.
One interesting characteristic of a megadungeon is that it might not have clearly defined boundaries: as the characters delve deeper, you just make it bigger. 
I personally have two distinct uses of the word “megadungeon”.
First, it may mean “huge dungeon”. In this sense, I consider the following megadungeons (this list is intentionally incomplete):
- Caverns of Thracia
- Dwimmermount
- Karak Azgal (WFRP 2nd ed)
- Khosura (published in Fight On!)
- The Castle of the Mad Archmage
- Tomb of the Bull King (Mazes & Minotaurs)
Second, megadungeon might refer to a the mythic underworld (as Philotomy describes it on his site), which differs from traditional dungeons in many aspects, including the fact that it is virtually endless in any dimension: a hundred or more rooms per level, countless levels and sub-levels, scores of means to change level, numerous entries and exits, wandering monsters of all kinds, etc. Basically, a framework, details filled in later as the players progress.
Funny thing you mention “Tomb of the Bull King”, as I wrote that module for “Mazes & Minotaurs”. It’s really great to see the module listed next to such great works, specially Caverns of Thracia :).
As you say, is a megadungeon by sheer size, although I’ve never considered it so “mega”. But the Tomb of the Bull King is also, in the “Mazes & Minotaurs” universe, an entrance to the Underworld, complete with shadows of the dead sadly walking in the corridors and going through the endless stairs at the center of the complex, down to Hades…
Damn, now I want to run it again :D.