My history in terms of AD&D isn't as impressive as seemingly the majority of people on the boards. I didn't even know there was a pen and paper AD&D (pathetic, i know). Anyway, what this surmounts to is a lack of understanding this "Undermountain". Where is it? What is it?
From what I've gathered its the most difficult thing EVER. Seemingly more ominous than the Abyss or Hell and far more difficult. Why does it exist? What is down there?
if it really IS the ultimate dungeon then it SHOULDN'T ever be conquered. Its sounds like it would take an entire army of demi-gods to take it on.
Is it Evil? Why are advanturers drawn to it? (are they all suicidal?)

Undermountain is a massive dungeon deep beneath Waterdeep, populated by all manner of foul and fearsome creatures, from all over Faerun and the Outer Planes as well. It is the creation of Halaster Blackcloak, a 30th level mage and one of the most powerful and crazy wizards in Faerun.

Why are adventurers drawn to it you ask? Why have adventurers ever been drawn to dungeons and adventure?? That's the answer!