If some dude would've told you 25 years ago that a significant chunk of the future heavy metal universe would take pride in being mere background clatter, there's a good chance you would have laughed in his mullet. Of course, all music often serves the purpose of background sound (i.e., we do stuff -- party, study, clean Grandma's garage -- while it's on), and from the psychedelic rock that metal largely evolved out of to Lou Reed's Metal Machine Music to Hawkwind-style space rock, there's probably always been music on metal's cusp that took that into account. At least into the early '90s, metal proper was "in your face" by definition; in the past two decades, though, that tendency seems to be slipping by the wayside. Metal might just be Muzak to fall asleep to, too!
This change of tune and/or philosophy came from at least a few different directions, most of which had something to do with metal getting druggier, more depressing, or both. Once you've re-embraced your inner Sabbath, as doom bands like Trouble and Saint Vitus did in the late '80s, acid rock and pysch and hour-long meandering plods through the desert with exotic fungi in your hemoglobin can't be far behind. Hence, the aptly named San Jose band Sleep's 1996-recorded/1999-released one-interminable-song album Jerusalem, one of the oldest selections tallied below. Even older is a release from converted Oakland punks Neurosis, who -- along with L.A.'s Isis, -- laid the groundwork for what eventually came to be referred to as shoegaze metal (or even "metalgaze," or just plan "NeurIsis metal"). It takes aimless oceanic clues from My Bloody Valentine and Cocteau Twins (and maybe Swans and Dead Can Dance and Radiohead), eventually emerging as a louder, noisier version of so-called "post-rock."
Then there's the black metal side of the equation, where gunked-up church-arson ugliness at some point opens up to haunting melodiousness; the guy most given credit for that is Norwegian neo-Nazi murderer and moron Varg Vikernes of Burzum, included here out of historical necessity, though please don't take his presence as an endorsement of either his views or his continued existence. He's just one more strain from which the 20 albums below likely emerged. Sweet dreams -- or nightmares, as the case may be.