The title pretty much sums it all up; I'd like all of us to share our favorite underground places to go for some nice XP boost or a chance to collect cool items.
There is a list of places where many people go for such reasons on Wolfe's Lair (Popular Hunting Grounds), but I'd like to have a collecting place for this community's favorites.
Mine is, by far, the burial chambers to the S-SW of Gloomoor. It is a somewhat large dungeon with large, open spaces and a LEGION of undead (Skeletons, Liches, Skeleton Mages and the occasional Zombies; some Ghouls and Poisonous Trolls, too). It means an awesome amount of XP, plus the tougher (champion) mobs tend to drop a set piece/unique every now and then.
Since it is not on Wolfe's Lair, I attach a pic of its location+a pic to show the density of enemies in the middle of the dungeon.

I usually ride around the Lower Underworld region, hunting Sakkara Demons and other enemies with high HP to cLevel ratios. I tend to get more Rune Drops here, at least in Multiplayer.

Can't remember the name of the place but the dark elf underground fortress was pretty good. It was fairly huge if I remember right so could spend alot of time down there.

Thanks for contributing, both of you!
@Ois
Indeed, Lower Underworld is one heck (almost literally ;-)) of a good place for leveling, good idea.
@Ralackk
You must mean Zhurag-nar (or, by its former, dwarven name, Gnarlstat). Now, THAT is one huge place.