What is UndeRground?

When we think of underground music, what artists are we thinking about? What sound or style of music are we thinking about? I remember when I was into hip hop (way back) it was easy to know what underground was. I feel like there was a definite understanding that underground meant underdog, usually unseen, unfiltered in there artistry, and usually untouched by the mainstream record label structure. A good example of an underground hip hop artist ( back then ) to me, was MF Doom. Even Doom was on Stones Throw though. Another question I would propose is: Does a label exist to further artist careers and to expand the reach of great artists? (that question may be for another day.) Now over to dance music. Today as we see dance music rise to the forefront in popular culture, does there exist an underground?