"The place was like a gigantic rabbit warren, with passages leading this way and that in every direction."
"We are now going underground! All the most important rooms in my factory are deep below the surface!"
"Why is that?" someone asked.
"There wouldn't be nearly enough space for them up on top!" answered Mr. Wonka. "These rooms we are going to see are enourmous! They're larger than football fields! No building in the world would be big enough to house them! But down here, underneath the ground, I've got all the space I want. There's no limit - so long as I hollow it out."
"The passages were sloping steeper and steeper downhill now."
All of this is on page 62 of my copy of Ronald Dahl's Charlie and the Chocolate Factory.
Mr. Wonka is surely a megadungeon designer. He:
- stocked it with diminutive workers with strange outfits and attitudes, since he couldn't trust the people who dwelled near his factory-lair
- pulled back from human contact to conduct further experiments
- and filled his enourmous underground factory complex with his special, secret works he couldn't share with anyone . . . until the time was right.
Then he uses tricks and guile to whittle down the visitors until only one remains, who he-copts into running the dungeon. We all know how that turns out. He does clearly label all the rooms, though, depending on misunderstanding and the flaws of the delvers to spring any traps upon them.
In my world, we call that a megadungeon run by an evil wizard. Only the loot is any different.