Tyler the Creator isn't underground anymore, but he raps to / about the underground nation. He isn't afraid that his music is classified as Horrorcore, nor do Brotha Lynch Hung.
Mars is another horrorcore rapper, who is nothing but underground. Good luck finding any of his shit though.
It seems like a lot of people are posting fairly well known rappers, who either have a major label contract, OR are youtube sensations. I listen to a lot of old hiphop that isn't necessarily popular anymore.