I don't know if you can really call these guys underground, and they're pretty old (2001) but this hip-hop duo is definitely underrated and their first and only album never got much mainstream success. However, it got very positive reviews from critics. Anybody ever hear of the Hip-Hop duo "Cannibal Ox"? They're on the Definitive Jux label which usually is home to many underground and alternative hip-hop artists (Like "Mr. Lif", "Del the Funkee Homosapien" and "El-P")
http://en.wikipedia.org/wiki/The_Cold_Vein
Their lyricism is simply amazing. It's clever, extremely complex, full of so much emotion, and it gives you chills every time you replay that song and hear it again. The songs are very dark in content to the point where they're pessimistic. The production is ill. Definitely an underrated hip-hop producer (El-P) or rather, one that just chooses to do little production work. Overall, definitely an album I think every hip-hop fan should listen to and I want to get the word out.
Some songs:
The F-Word
http://www.youtube.com/watch?v=IljbLomFccA
Iron Galaxy
http://www.youtube.com/watch?v=9DVNwyCymUw



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