Most of what you need to know about Mastodon’s music is summed up in its name: gigantic guitar riffs, earth-shaking bass lines, stomping double-time rhythms, roaring vocals. Everything these heavy-metal monsters do is writ large, and its lyrical topics cover big things—literally: The Atlanta quartet’s second album, 2004’s Leviathan, was a concept record based on Moby Dick; the new one is about climbing the titular crag, a mythical, beast-filled place fit for a Tolkien novel.

But Blood Mountain isn’t simply a fantasy tale about humping it up a hill while encountering creatures like the Cysquatch (a one-eyed Sasquatch, natch), or the antlered wolf-moose-demon that’s pictured on the album cover, any more than Leviathan was a simple story about a whale. That record came out on influential indie-metal label Relapse, and its lyrics were a metaphor for a band on a long quest through hostile lands, touring in a stinky van and playing skanky rock clubs, all the while longing for home. It sold about 100,000 copies, a huge number for an independent label, and received mainstream press raves from the likes of the New York Times.

With Blood Mountain, both Reprise and the band hope to capitalize on Mastodon’s remarkable underground success. Perhaps that’s why Mastodon brought in guest singers for Blood Mountain, even though they don’t add much. Maybe the band just did the hip-hop thing and called on some homies—or maybe the homies just barged in, so fervent was their desire to get down with metalheads actually flirting with the mainstream. Guest vocalists include Neurosis’ Scott Kelly on the pummeling “Crystal Skull,” and the Mars Volta’s Cedric Bixler-Zavala visits on the Wagnerian “Siberian Divide” to help sing lines like “Frostbite eating away at our limbs/Slit the belly Brontotherium.” Meanwhile, Queens of the Stone Age’s Josh Homme adds his vox to the operatic “Colony of Birchmen” and reads a fake fan letter on the ignorable hidden track that follows the album-closing, Ozzy-like meditation “Pendulous Skin.”

The band plays diffident, uncompromising music more suited to nerds than to the herd, but like System of a Down, Mastodon often gives momentary breathers in its music, like sung choruses and moody guitar interludes, to give listeners a chance to catch up. The group doesn’t make listener-unfriendly music; you just have to meet the band halfway. Mastodon has managed to attract both effete indie kids and hardcore headbangers in black T-shirts featuring fantastical airbrushed paintings. Fittingly, then, while the words barked by bassist-vocalist Troy Sanders and guitarist-vocalist Brent Hinds concern “ogres and dwarves,” “elephant spirits,” and an “Ice God,” the song cycle also works as a meditation on maintaining one’s identity and ideals while facing larger expectations.

The first single, “The Wolf Is Loose,” isn’t merely about a shape-shifting lupine coming from another world to wreak havoc on an unsuspecting populous. It starts out a little like Van Halen’s “Hot for Teacher” before unleashing the lines “The hero of the gods/The crossing of the threshold/The belly of the whale/Refusal of return.” It could easily refer to Mastodon’s jump to the big leagues. Or it could be some straight-up Dungeons & Dragons shit, I dunno.

But ruminating too long on Mastodon’s lyrics misses the point. The men of Mastodon play their instruments like jazz musicians who, well, aren’t pussies. Drummer Brann Dailor is especially skillful, and his speedy fills and blustery polyrhythms are nicely complemented by Hinds and Bill Kelliher’s torturous riffs. The typical Mastodon song is puzzled together—with sledgehammers—using pieces that don’t necessarily fit. Circles are mashed into squares; triangles are—well, fuck triangles: Mastodon hacks off the tops, makes them into trapeziums, and punches them into pentagons.

But tracks like “Sleeping Giant” and “This Mortal Soil” feature minor-key, delicate finger-picking and pastoral psychedelia to soften the sledgehammers’ blows—there’s only so much pummeling even a committed maggot can take. Indeed, for a band that’s easily characterized (see: name), Mastodon makes music that’s remarkably textured. “Capillarian Crest” has so many changes of tempo and mood that you need a GPS to follow it, like a gruffer Mars Volta, or maybe Coheed and Cambria with a guy singer. “Hunters of the Sky” starts slow and then becomes a collection of ever more unrelenting and impossibly brutal riffs, and “Hand of Stone” features grandly sliding guitar chords over a galloping, rocking-horse rhythm then gives away to double-kick-drum bombast in the chorus and twin-lead segments.

The most extreme example of Mastodon’s dexterity is the instrumental “Bladecatcher,” which starts with an unaccompanied African-type-guitar riff before moving into twin-lead psychedelia and then fierce speed metal topped by spasmodic ax-grinding that mimics turntable-scratching. Like all of Blood Mountain, it’s demanding music, akin to slogging up a hillside that’s dominated by monsters. But once the beasts have been banished, and the CD stops spinning, a feeling of euphoria bubbles up. Mission accomplished, peaks claimed, asses kicked.