First, recognize this: underground rap is not like indie music. While both categories of music exist under the radar of the mainstream, the underground movement is no mere alternative; it is the redemption of an entire genre. Sophisticated, innovative, and genuinely insightful, underground rappers are able to lay down the autotuned tropes of so-called "modern" rap in favor of pursuing higher artistic goals. Despite a trade off in profitability and mass appeal, the result is a wave of diverse new music that breaks neatly away from the homogeneous garbage that seems to receive so much air time on the radio.
Indeed, one might even call it a new frontier. Underground rappers are known for collaborating with one another, mixing and merging styles into unique and distinct new blends. In this same spirit of synthesis, underground will often draw from other genres beyond hip-hop, including such seemingly obvious additions like funk and reggae but also reaching out into free-form jazz and, in the case of Sage Francis, country. Individual rappers will each cultivate unique mixes of influence, staking out audible identities in a way that not only sets them apart from their compatriots but actually encourages them to delve deeper into the musical unknown in the spirit of friendly competition.
Of course, for every Sage Francis and MF Doom, there are Chali 2nas and Roots Manuvas - rappers whose styles do not so much push forward as fall back. Instead of pioneering, they seek to emulate the style of the early 80s prior to the invasion of West Coast style gangsta rap. While this may seem counter-productive to the concept of underground it is actually essential to it; what these artists lack in musical variety they make up for in technical and lyrical excellence. It is in their hands that rap verse is exploited to its fullest extent, conveying complex narratives in an easily understandable form. They also expand beyond the typical urban setting, exploring the religious, the western (as in the Wild West), and even outer space. Because of the collaborative nature of the underground community, such artists will often diffuse their talents to others. In fact, a great lyricist like Gift of Gab can be (and currently is) used to anchor several different groups of rappers.
The result of all this is an outpouring of crisp, refreshing verse coupled with strange and wondrous new styles that departs whole-heartedly from the tired and bland gangsta style that gives rap such a bad name today. Even if you've hated rap for as long as you can remember, I can only suggest you try it. Here are a few artists to start with:
N.A.S.A. - I highly recommend you listen to their song, "People Tree". Really. Do it.
Deltron 3030 or Del Tha Funky Homosapien
Talib Kweli
Lateef the Truth Speaker
Vursatyl
Ohmega Watts
Gang Starr - Major proponent of jazz rap.
MF Doom and associated collaborations Dangerdoom and Madvillain. Very strong jazz rap.
Boots Riley
Jurassic 5 - Chali 2na's old group, now defunct but still worth a listen. Very retro sort of sound.
More to come as I think of them.