Underbelly is often used to refer to the hidden part of almost anything. Often prefixed with the word seedy. "In the seedy underbelly of Redhat, the programmers pumped out perl code to take over the world."
I must note however that no one ever says the word underbelly out loud. I have seen it in print 1000 times. But I have never heard the word come out of anyone's mouth. | 
