Mastodon are a bunch of doom–haunted, myth–obsessed, meat–and–potatoes Southern badasses who have become the most important new band in metal. They awed the underground with 2004's massive, Moby Dick–themed Leviathan; then, in 2006, they grabbed the crown with the brutally psychedelic, ogre–hunting Blood Mountain. It established them as a rare beast: a band respected by diehards for its technical bonafides but also by indie rockers for its jump–cutting conceptualism. Mastodon are a full–on metal band for people who think they're too cool to like metal.

With Crack the Skye, their fourth record, the Atlanta quartet have set out to meet the masses for their Metallica moment. They've moderated their relentless assault — a bit — and joined forces with hard–rock producer Brendan O'Brien (Bruce Springsteen, Pearl Jam). There will be naysayers among the band's extreme, tatted legions. But Crack the Skye is an awesome display.

The new sound is summed up in the 13–minute prog–rock epic "The Last Baron." It begins with steely guitar–picking and ominous swirling chords, then explodes into a midtempo march with wildly eddying drums and — for these Godzilla growlers — some startlingly melodic vocals. And it keeps exploding: a twin–guitar firestorm, dub tsunamis and rapid–fire, on–a–dime rhythmic shifts, all churning beneath lyrics that suggest a soul watching the apocalypse. "I was standing staring at the world," howls Brent Hinds. "I still can't see it." It's thrill–ride stuff that conjures not just art–metal predecessors such as Helmet but also old–school prog–rockers like Rush, Frank Zappa and Yes.

"The Last Baron," like much of the set, was written while Hinds recuperated from a head injury that left him with vertigo, the result of an incident in Las Vegas following the 2007 VMA's. Here, expressions of pain and clouded vision ricochet between metal brutality and art–rock uplift. "Punching these holes in my head," goes a verse on "Quintessence," which is built on hardcore outbursts and dreamy synth and vocal billowing. The title track suggests a no–joke battle with the devil, featuring talk–box–style effects and soaring, near–pop–metal vocals. At one point, Hinds even busts out a banjo.

Skye is a blur of themes. It's the group's "ether" record, following the fire–water–earth sequence of its first three, so there are unmoored souls moving through space. And there's a narrative conjuring Russian visionary Rasputin in "The Czar," the record's four–part centerpiece. O'Brien's production gives the group a vocal presence it's never had before: With harmony choral bits recalling System of a Down, flashes of grandeur and even some pop schmaltz, "The Czar" is a 21st–century prog–metal classic.

Ultimately, Mastodon are less about basement bong–circle storytelling than about extreme–sports musicianship, whether it's drummer Brann Dailor's churning toms and cymbals, Troy Sanders' rushing bass runs and keyboard atmospherics, or the hydra–headed guitar outbursts of Hinds and Bill Kelliher. Mastodon are less willfully eclectic than the Mars Volta, less high–concept than outfits like Pelican. But by bringing more strains into their ass–kicking collage metal, they've opened a whole new range of possibilities.