I'm not really deep into theory of jazz or anything, in my understanding "Hard Bop" (or "Hard Bebop") is more intense, rhythmic, and has significantly extended solos compared to just "Bop" (or "Bebop") Jazz. There is more about this on wiki, but I personally find it easy to hear the difference intuitively after hearing some bebop and some hard bop works. The main source to dig for classics of this genre is a label called "Blue Note", but it's huge and features a lot of other styles of jazz. I personally discovered this genre from a work called "Bolivia" by Cedar Walton. It's also hard bop, but it's not guitar driven.
I'm not entirely sure it would strictly qualify as "underground" in absolute sense, because Wes Montgomery is one of the bigger names in the genre, but nowadays those trickier styles of jazz like hard bop, post bop, modal and free jazz are definitely something most people never knew existed. So I think it would be nice to post this kind of stuff periodically.
I couldn't write it better than wikipedia: Math rock is a style of alternative and indie rock ... characterized by complex, atypical rhythmic structures (including irregular stopping and starting), counterpoint, odd time signatures, and extended chords.
Does it look different to you or do you mean text posts?
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For guitar stuff specifically, check this one.