Coolest Underground Dungeons

[Psion]

No. I am interested in the Dungeon, with it's inhabitants, echology and natural dangers&features.

I'll second Tomb of Abysthor, then if you are not afaid of something fairly sizeable. It's an interesting little jaunt, though it can be a bit challenging. Also note, it's for 3.0, so ouy may need to do some updates.