*** 1/2 MASSIVE ATTACK, “Mezzanine,” Virgin

Massive Attack’s 1991 debut album, “Blue Lines,” has become a watershed album in the history of electronic dance music for a number of reasons. By melding ominous dub bass lines with neatly patterned drum loops and richly atmospheric production, “Blue Lines” virtually invented trip-hop.

But it was the emphasis on composition, as evidenced on its dance-noir masterwork “Unfinished Sympathy,” that provided the collective from Bristol, England, with its entry point into the mainstream and spawned (for better or worse) such contemporary bands as Sneaker Pimps, Olive and Mono.

“Mezzanine,” which is only Massive Attack’s third album in eight years, delves even further into the kind of down-tempo, introspective head music found on the band’s sophomore effort, 1994’s “Protection.” Stealthy bass lines and half-cocked beats wend through densely constructed tracks that slowly reveal themselves over time, like lapsed-image snapshots. All of the tracks exist in a hazy, mysterious murk; only the occasional fugitive vocal, from the Cocteau Twins’ Elizabeth Frazier or reggae singer Horace Andy, emerges to provide a focal point.

“Mezzanine” is so intent on maintaining a relaxed pace that it’s a bit unnerving after a while, but that’s where it derives its power; like a particularly vivid dream, it will haunt you long after you’ve set it aside.

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