Vertebrae offers further proof that Enslaved have found their groove as a post-black metal proposition: content to carry on testing the boundaries of their relatively exclusive and self-ordained domain, where progressive and psychedelic influences find unlikely sympathy with the extreme musical foundations of yesteryear (which, to the group's immense credit, have not been summarily abandoned).

However, at first it seems that Vertebrae will be something of a stylistic holding pattern in relation to acclaimed predecessors Ruun and Isa (even retaining the same exact lineup, for a change), before eventually revealing itself, over prolonged exposure, to indulge in just as much experimentation.

In broad terms, this means that the recurring dynamic contrasts between sheer violence and exquisite splendor still account for most of the thrills delivered by these multifaceted and frequently unpredictable songs.

But, more specifically, those experiments yield numerous creative breakthroughs, such as the flashes of Pink Floyd worship on "Ground," the jazz-inflected solos found in "Reflection," and the art rock minimalism (à la Tool) heard on "Center," to name but a few.

Meanwhile, Vertebrae probably contains an even higher percentage of -- and greater comfort with -- clean vocals on the part of frontman Grutle Kjellson, whose voice is often layered to dense harmonies with those of his bandmates, to wonderful effect.

Moreover, Kjellson's lyrical collaborations with the group's guitarist and primary songwriter, Ivar Bjørnson, frequently achieve unprecedented heights of inspiration and clarity on tracks like "Clouds," "Reflection" (again), and the surprisingly optimistic "New Dawn," where existential philosophy and blood-drenched mythology commingle as effortlessly as the aforementioned musical elements (i.e., Viking metal purists will not feel shortchanged).

And for all of these reasons, Enslaved continue to represent the absolute evolutionary cutting edge of extreme metal, delivering in Vertebrae yet another spectacle of imagination and quality control, matched only by Sweden's equally consistent Opeth.