Underground versus mainstream

[The Sleeper]

Hendrix, The Doors, Joplin, The Beatles(almost all of them are underground) so on and so on.

I could be wrong, but I think the term "underground" refers to obscure music that is not played on radio or MTV and people have to actively seek out. Everyone knows the Doors, Joplin, Hendrix, etc. I wasn't around during that era so maybe they started off underground and when everyone realized how good they are they became popular. But at this point, I would consider those groups mainstream (especially the Beatles).