I have tried to make sense of the map at the following URL, but can not read it effectively:
Is there an good map of the Mazes of Menace (Nethack) ?
I agree it is pretty hard to read, and I think is better explained in text.
You start on Dungeon Level 1. At certain points in the game you can branch off to another area.
As you can see, due to it's complex design, and random generation, it is very difficult to produce a good map of the Mazes of Menace
(All information has been collected from Wikihack)