Traveling from Czarist Russia to deep space and back again, Crack the Skye is a conceptually ambitious ride to the outer reaches of progressive metal. Along the way, Mastodon unload pure heaviness. In just under 11 minutes, “The Czar” blends cosmic psychedelia and crazy-muscular sludge riffs. Far shorter, yet no less ambitious, “Divinations” opens with old-time banjo before cutting to screaming twin guitars and some of Brann Dailor’s most furious drumming. Needless to say, the rest of the album is just as powerful.