From the Underdark sourcebook there is a page that shows a map of the Earthroot with many cities under Thay that are unlabeled. There also appears to be a city under the eastern edge of the Thesk mountains. Does anyone have an idea of what is supposed to be there?
Drizzt's Guide to the Underdark sadly doesn't get further east than the Dales. But the 3e Underdark book (which isn't nearly as good, but does have its moments) is where you'll find a description of that many-segmented city.
Doggedly converting 3e back to what D&D should be... Sigh... And now 4e as well.