Few acts get to define a genre, much less direct it. Since its 1991 debut Blue Lines, Massive Attack has been synonymous with trip-hop, even with its passing. The band last surfaced in 1998 with Mezzanine, an album that seemed to take the mongrel style's fusion of breakbeats, soulful vocals, ingenious samples, and electronic atmospherics to its paranoid extreme, just as the sound was reaching the saturation point. It would have been a logical time to bow out. Since then, the group has weathered the departure of one key member (Adrian Vowles, a.k.a. Mushroom) and a scrapped record that led to the long-term absenteeism of another (Grant "Daddy G" Marshall). Those developments essentially left Robert del Naja, sometimes known as 3-D, to record 100th Window alone, and left fans with every reason to expect a disaster instead of a recording every bit as compelling as the rest of Massive Attack's discography. Where Mezzanine sounded scared and sinuous, 100th Window goes a step further: It sounds like music made for headphones with teeth. Even after shedding members faster than Robert Pollard, Massive Attack has preserved its reputation for consistency and daring. It has many disciples, some of whom are excellent, but no one has done so well at mastering the tension between the sensuous and the cold, the frightening and the alluring. The album's tracks, as usual, take time to unfold, establishing a mood and then turning the screws, slowly intensifying to a kind of breaking point. Alternating his vocals with contributions from frequent Massive Attack collaborator Horace Andy and Sinéad O'Connor (a natural pairing as good in the execution as it sounds on paper), del Naja blows that tension up to global proportions, tapping into a sense of general uneasiness and giving it a soundtrack. Though never explicitly political, even on a track like "A Prayer For England," 100th Window is shot through with a distrust of the powers that be, and a sense of foreboding for a future that follows the present course. Downbeat in every sense, the album carries reduction of a sound to its abstract principles to an absurd degree, winding down with a pattern of notes that repeats until the CD hits the 74-minute mark. That would sound like another dead end, if not for the rumors promising Marshall's return and another record by year's end, featuring collaborations with Mos Def and Tom Waits. As with all Massive Attack matters, it seems best to simply shrug and wait for the next revelation.