Which underground area do you like the most?

I'm far too prone to claustrophobia in real life to attempt any sort of spelunking (plus I saw The Descent, which solidified that feeling), but I'm the opposite in MMOs. I love exploring underground regions in games, whether they be little hidey-holes for Hobbits or grand cities that never see the sunlight.

As of late, I've come to greatly appreciate the wide variety of caves and caverns in Guild Wars 2, all of which manage to be interesting without feeling restrictive or overly rock-themed. It feels like we've come a long way from some MMOs' ideas of underground usage (I'm looking at you, City of Heroes, and your oh-so-bland caves).

Obviously, the topic du jour is MMO underground areas and which ones you like the most. Lay it on us where the sun doesn't shine, OK?

TH