Underground hip hop beats simply define the genre: hard-hitting, epic, and a bit more challenging to get into. Underground rap is qualified as any style of rap music outside of the standard commercial boundaries associated with popular rap and hip hop music. It is defined by heavier and more intense lyrical content, gritty and abrasive beats, and an all-around darker personality. Those who thrive on mainstream rap may shy away from such a concept, but those with open minds will discover a very satisfying listener experience.
Artists that simply represent the early upbringing of underground rap music range from A Tribe Called Quest, to Del Tha Funkee Homosapien, to Jedi Mind Tricks. All represent different niches in the hip hop world.
And that's the interesting thing about underground rap: artists don't have to limit themselves by rapping about any one thing in particular. The outlet provides them more freedom to get creative. For example, there was 2000's super-group, Deltron 3030, which combined the personas of Del Tha Funkee and Dan The Automator, spinning an ambient tale about the future. There are also sub-genres of the sub-genre, leading to musical styles such as nerdcore, a genre specializing in its focus on nerd/geek culture. Examples of this subgenre are MC Frontalot, The Wordburglar, and Optimus Rhyme.
Underground hip hop beats lack what makes mainstream music so overwhelmingly popular: accessibility. The melodies are constantly changing, drum patterns may be off, and the instruments may seem shrill to the ears at first. After several in-depth listens, however, listeners will become one with the music, and the journey that it represents, and will fully understand the artistic integrity of the rappers and producers.
Ultimately, underground hip hop beats are what help push the lyricism to the forefront and make the music what it is: epic, challenging, and rewarding.