Hussar
Legend
Oh yeah. @Dyson Logos is a GOD when it comes to megadungeon design. Definitely a master class in dungeon design if there ever was one.You folks should look up the work of Dyson Logos. He has created some tremendous mega-dungeon maps, many of them multi-levels.
His opus, if playing with 5 feet = inch scale, would be 26 feet long if printed. He admits it is impossible to use, but the inspiration.....
There is also the One Million maps by The One Million Info by Zatnikotel on DeviantArt Zatnikotel. Fabulous stuff. Particularly if you are like me and want to run on virtual tabletop. Beautiful stuff.
Christopher West of Dungeon Magazines Map's of Mystery is another fantastic go to source. The Map Store
Really, if you want a megadungeon map, you really are spoiled for choice. There's TONS of stuff out there.