Megadungeon

The traditional "dungeon" in D&D, video games, and fiction is often portrayed as the stronghold of an evil wizard or dark lord.

From Donjon by Sfar+Trondheim

Castlevania: Dracula X for SNES

Sometimes they are remnants of a lost civilization.

Yet despite their differences, it is presumed that all of these dungeons were built for some specific reason, and through the efforts of manual labor.

However, when it comes to megadungeons, there are many different thoughts regarding their nature since they do not always follow the type of logic that is referred to as Gygaxian "Naturalism".

DM David gives a nice overview of the subject, and mentions the influential article about mythic underworlds from Philotomy's Dungeons & Dragons Musings.

So where do these megadungeons come from? I image that the more mundane megadungeons are just much larger versions of standard dungeons.

Megadungeons such as Utumno from The Silmarilion seem to have been built just like a normal dungeon (albeit with the resources available to a deity).

Utumno by Ian Miller

Similarly, machines like the Builders (kensetsusha; 建設者) from Blame! can continuously build structures, to the point where they can entomb an entire planet.

From NOiSE

From Blame!

The living dungeon is also a concept that has been used in fiction. Typically, they are explained as a [super]natural phenomenon that appears in worlds where magic exists.

In DanMachi, the Dungeon is inimical to mankind, and the inhabitants are viewed as dangerous and irrational by the humans that seek to plunder the dungeon of its treasures.

Living dungeons in Dungeon Meshi on the other hand, are seen as more of a natural phenomenon. At least some dungeons can be controlled by the person who assumes the role of  lord of the dungeon.

The final type of dungeon that is closest to the mythic underworld is what I would consider the world within a witch's barrier (majo no kekkai; 魔女の結界) from Puella Magi Madoka Magica. The labyrinth (迷路) within the barrier seems to be created entirely from magic.

In the world of Madoka Majica, witches are created when magical girls reach peak despair. The FRPG world equivalent would be when magic users reach Wizard's Twilight (to use Ars Magica terms).

GERTRUD

As for actual mythic underworlds, I think of them more as the locations where HeroQuests take place, not places where adventurers go merely to kill things and aquire currency.

Palace of the Silver Princess

Palace of the Silver Princess was originally the project of Jean Wells, and later rewritten by Tom Moldvay. The module was, and continues to be embroiled in controversy for a number of reasons. I do not know the truth of the matter, and can only present some the information that I have been able to find on the interwebz along with some of my own commentary.

• An article from the Acaeum that details the events surrounding the creation of the module, and the editorial complaints against the original version.
• Podcast interview of Jean Wells at Save or Die!
• Part deux of an interview with Jean Wells at Grognardia, which describes some of the events surrounding the development of B3 from the view of the author.
• Some information about Duchess and Candella, two of the NPCs found in the module.
• A post from Old School FRP on some of the differences between the orange and green cover modules.
• A short paper by Chris Danielson at acadamia.edu that proposes some theories about why things turned out the way they did for Jean Wells.

It has been claimed that Palace of the Silver Princess was known internally as Phallus of the Silver Princess, but it is not clear how widespread this name was used, or why it was selected.

Was it because "palace" rhymes with "phallus" (to this day, I know gamers who make up silly names like this to make sport of other players), or was it because of the artwork in the module?

Adventurer offering a bear cub his "meat"?

Statue of t-girl replaced with statue of dragon?

The issues with the artwork seem more like people seeing things that are not actually there because of their own hang-ups, but to this day I read comments claiming how the author (Wells) had some sort of weird [sexual] obsessions - most likely from people who did not even read the original module or see the "offensive" pieces of art (many of which were not even drawn by Wells).

Finally, the various puritanical comments quoted in Danielson's paper by various TSR staffers seem rather ridiculous, since a year after the publication of the sanitized B3, TSR went on to approve production of a children's toy that looked like this:

Is that a dagger in your belt or are you just glad to see me?

If someone was offended by the Roslof or Wells illustrations,
what would they make of these?