i know some of you have seen some of this stuff before, but i thought it was worth a repost. from the quite ace BLDGBLOG, here's an article on underground cities in Turkey:

http://bldgblog.blogspot.com/2007/08/derinkuyu-or-allure-of-underground-city.html

and here's one a bit more London-centric for people who havent seen it yet:

http://bldgblog.blogspot.com/2005/11/london-topological.html