The Meaning of Underground

In class we talked about various interpretations of the word "underground."
For example, it could imply someone who is observing or spying.  It could also mean someone who is considered "other" and therefore is reclusive and isolated, or even alienated (both physically and philosophically) from society.  Underground Man certainly seems alienated, with his dark and cynical view of humanity.

I also couldn't help but think of other meanings of the word "underground."
It can also mean "secret," or "relating to a social and artistic world that is different and separate from the main part of society," as well as "a group or movement organized secretly to work against an existing regime" (a bit reminiscent of the Petrashevsky circle that Dostoevsky joined).

I was also thinking about the Underground Railroad, the secret network designed to help slaves escape to the free states in the 1800s in the United States.  It probably sounds kind of unrelated to Underground Man, but I think some of the same themes are echoed, such as the idea of free will.  It seems that the term "underground" is often associated with people's differing ideas on freedom and some groups' or individuals' reactions against the oppression of freedom.  Thoughts?