Mystery burns at the heart of Portishead, lurking deep within their music and their very image.

From the outset they seemed like an apparition, as if their elegant debut, Dummy, simply materialized out of the ether in 1994, as their stately blend of looped rhythms, '60s soundtrack samples, and doomed chanteuse vocals had only a tenuous connection to such Bristol compatriots as Massive Attack and Tricky.

Soon enough, Portishead's unique sound was exploited by others, heard in swank clubs and high-end dinner parties on both sides of the Atlantic, a development that the trio of Geoff Barrow, Beth Gibbons, and Adrian Utley bristled at instinctively, recoiling into the darker corners of their sound on their eponymous 1997 sophomore album before fading back into the ether leaving no indication when they were coming back, if ever.

They returned 11 years later, seemingly suddenly, with Third, supporting the album with candid interviews that lifted the veil from their personality, yet the mystery remained deeper than ever within their gorgeous, unsettling music.

That strain of uneasiness is a new wrinkle within Portishead, as in the '90s they favored a warm, enveloping melancholy, a rich sound that could be co-opted and turned into simple fashion, as it was by band after band in the heyday of the swinging '90s.

So many groups grabbed ahold of Portishead's coattails that it's easy to forget that in 1994 there was no other band that sounded quite like Portishead -- not even Massive Attack and Tricky, who shared many surface sounds but not a sensibility -- and that is just as true in 2008, years after trip-hop has turned into history.

Their cold, stark uniqueness isn't due to a continuing reliance on the cinematic textures of Dummy, although there are echoes of that here on the slow-crawling album openers "Silence" and "Hunter," songs just familiar enough to act as reminders of how Portishead are special, yet just different enough to serve notice that the trio is engaged with the present, even if they've happily turned into isolated recluses, working at a pace utterly divorced from the clattering nonsense of the digital world.

Third is resolutely not an album to be sampled in 30-second bites or to be heard on shuffle; a quick scan through the tracks will not give a sense of what it's all about.

It demands attention, requiring effort on the part of the listener, as this defies any conventions on what constitutes art pop apart from one key tenet, one that is often attempted yet rarely achieved: it offers music that is genuinely, startlingly original.

Surprises are inextricably intertwined throughout Portishead.

There are jarring juxtapositions and transitions, as how the barbershop doo wop of "Deep Water" sits between those twin towers of tension of "We Carry On" and "Machine Gun," the former riding an unbearably relentless two-chord drone while the latter collapses on the backs of warring drum machines.

Echoes of Krautrock and electronica can be heard on these two tracks, but that very description suggests that Third is conventionally experimental, spitting out the same hipster references that have been recycled since 1994, if not longer.

These influences are surely present, but they're deployed unexpectedly, as are such Portishead signatures as tremulous string samples and Utley's trembling guitar.

Out of these familiar fragments from the past, Portishead have created authentically new music that defies almost every convention in its writing and arrangement.

As thrilling as it is to hear the past and present collide when "Plastic" is torn asunder by cascading waves of noise, Third doesn't linger in these clattering corners, as such cacophony is countered by the crawling jazz of "Hunter" and the sad, delicate folk of "The Rip," but a marvelous thing about the album is that there's no balance.

There is a flow, but Portishead purposely keep things unsettled, to the extent that the tonal shifts still surprise after several listens.

Such messiness is crucial to Portishead, as there's nothing tidy about the group or its music.

Experimental rock is often derided as being cerebral -- and this is surely enjoyable on that level, for as many times as Third can be heard it offers no answers, only questions, questions that grow more fascinating each time they're asked -- but what sets Portishead apart is that they make thrillingly human music.

That's not solely due to Gibbons' haunting voice, which may offer an entryway into this gloom but not its only glimpse of soul, as the perfectionism of Barrow and Utley have resulted in an album where nothing sounds canned or processed, the opposite of any modern record where every sound is tweaked so it sounds unnatural.

Third feels more modern than any of those computer-corrected tracks as the group's very sensibility mirrors the 21st century, where the past is always present.

Then, of course, there's that rich, fathomless darkness that Third offers, something that mirrors the troubled days of the new century but is also true to that shimmering, seductive melancholy of Dummy.

Here, the sad sounds aren't quite so soothing, but that human element of Portishead gives them a sense of comfort, just as it intensifies their sense of mystery, for it is the flaws -- often quite intentional -- that give this an unknowable soul and make Third utterly riveting and endlessly absorbing.