Rule #1 : You don't talk about the underground.
Rule #2: You bring cookies to secret underground meetings, no exceptions.
The underground is any magician who practices, lives and breathes in his basement. Or...it's what D ICE R said, just a magician/move/routine that hasn't been seen by many other magicians. The Clipshift was used for 10 years 'in the underground' before being released via DVD.