Though some quality bands have produced underground metal with a substatial level of "formal" theory in mind (At the Gates, Sacramentum, Gorguts, etc.), the real marvel of this music is that it's created by intelligent but generally untrained musicians who produce coherent and evocative music out of a subconscious INSTINCT towards order. A good example of this is Darkthrone (think UaFM) who were attempting to create deliberately alienating, ugly, and random sounding music, but shat out a grippingly beautiful hymn to darkness, maybe despite themselves. To me, this element of surprise, when order escapes the swirling chaos against all odds, is the most important and consistent attribute of quality underground metal, and the one that separates it from other worthwhile musical mindsets ("genres") like classical or ambient, and so justifies the chaotic and ugly aesthetic of metal, as opposed to the clean tones common to most quality western music. You can't transcend chaos without the chaos.