I've prepared a partial map of the first level:
To see a larger image, click here: IMG_1357.jpg
This is just a third of the first level. Once they get to room #31, they flip a switch to open a gate to the next section of the dungeon, and blammo! Klaxons go off, loud grinding noises echo all over the dungeon, and outside, entrances that have been buried for centuries suddenly burst open, and floodlights illuminate the sky. Every creature for miles will know something is up and come to investigate.
And thus, the megadungeon's weird-fantasy inhabitants, lurking in the dark for untold years, get to meet the goons from the outside world. Humanoids, wizards, dragons, and others will flood the place, looking for loot. So the players will have to contend with outside threats as well as dungeon weirdness as they work their way deeper.
Their big advantage is that their entrance isn't lit up like the sun, so unless they blow it, their entrance will be their little secret.