Look through the ground sometime, you'll see miles of water and waves being rendered.
http://imageshack.us/f/689/watervl.jpg/
WoW had this problem in beta, and made the underground nothing but white. Solved a lot of FPS issues.
And don't bother asking how i got under there, i just finished mining and walked right through a wall. It's perfectly logical why places like the Hall's of Flames have zero fps lag, it's because they're new and do not have this awful design flaw.
Furthermore, I really can't believe they would do something like this in their own game.... its quite amateur design.