Heavy metal is a musical genre that's often maligned by critics and mainstream listeners as being stoopid and all flash, no substance. But hardcore fans -- even casual ones like me -- find music that's heavily layered, lyrically complex, physically complex, and ultimately, challenging.
VH1 Classic's "Metal Evolution" is an excellent series that documents heavy metal and hard rock, and well worth watching. Of particular worthiness is Episode 1, in which host Sam Dunn traces the roots of metal and hard rock to classical, blues, even opera music.
It's a fascinating study suggesting that in its base form, heavy metal isn't too disconnected from "heavy music," as classical music often strives to be.