Just in case you don't keep up with The New York Times' metal coverage, let's recap: Sunn O))) is a guitar duo that performs in hooded robes and specializes in doom-laden dronescapes inspired by Earth's agonized, tonal trawl. The two operate the Southern Lord label, to which chameleonic Japanese metallers Boris are signed. Collaborations between bands often wind up as musical games of King of the Hill, with each entity battling for dominance. The only track on Altar on which Boris and Sunn O))) attempt to synthesize their individual aesthetics is opener "Etna," with the former's psych-metal riffs sprouting from and hovering above the latter's dirgelike solemnity. After that, all bets are off. "Fried Eagle Mind" comes off like Sonic Youth under the influence of some primo bud, and "The Sinking Belle," a goth-as-frick number, is stage-whispered by Jesse Sykes, pianos and pedal steel spiraling and splintering away behind her. Earth's Dylan Carlson, Soundgarden's Kim Thayil and many others appear on this genre-blind revelation, begging a question: Wouldn't it make more sense to give this project its own name and make it an ongoing concern?