First off, a warning: the best way to encounter Mastodon's Crack the Skye for the first time is with headphones.

Reported to be a mystical -- if crunchy -- concept record about Tsarist Russia, this is actually the most involved set of tracks, both in terms of music and production, the band has ever recorded.

"Ambitious" is a word that regularly greets Mastodon -- after all, they did an entire album based on Moby Dick -- but until now, that adjective may have been an understatement.

There is so much going on in these seven tracks that it's difficult to get it all in a listen or two (one of the reasons that close encounters of the headphone kind are recommended).

It may seem strange that the band worked with Bruce Springsteen producer Brendan O'Brien this time out, but it turns out to be a boon for both parties: for the band because O'Brien is obsessive about sounds, textures, and finding spaces in just the right places; for O'Brien because in his work with the Boss he's all but forgotten what the sounds of big roaring electric guitars and overdriven thudding drums can sound like.

The guitar arrangements on tracks like "Divinations" and "The Czar," while wildly different from one another, are the most intricate, melodically complex things the band has ever recorded.

There are also more subtle moments such as the menacing, brooding, and ultimately downer cuts such as "The Last Baron," where tempos are slowed and keyboards enter the fray and stretch the time, adding a much more multidimensional sense of atmosphere and texture.

Still, Crack the Skye rocks, and hard! Its shifting tempos and key structures are far more meaty and forceful than most prog metal, and menace and cosmological speculation exist in equal measure, providing for a spot-on sense of balance.

Some of the hardcore death metal conservatives may have trouble with this set, but the album wasn't recorded for them -- or anybody else.

Crack the Skye is the sound of a band stretching itself to its limits and exploring the depth of its collective musical identity as a series of possibilities rather than as signatures.

And yes, that is a good thing.