On its latest album, "No Line on the Horizon," U2 sounds like a band trying to shrug off years of staleness.

The Irish quartet is once again in transition, uncertain of its destination, and producing some fascinating music along the way. Five of the 11 tracks sound as fresh as anything U2 has done in a decade.

The rest aren't nearly that good, putting this in the middle tier of the band's dozen studio releases. But at least the band is trying to reconnect with the sense of yearning and mystery that once made it special.

It's hard to be mysterious when you're the biggest band in the world. But that sense of mystery is key to U2's sound; the band's fondness for atmosphere forged a new kind of stadium rock in the '80s, epic yet somehow intimate, then embraced even deeper undercurrents on the '90s masterpiece, "Achtung Baby!"

That adventurousness was largely missing from the previous two U2 studio albums, "All That You Can't Leave Behind" (2000) and "How To Dismantle an Atomic Bomb" (2004). These were "back to basics" gambits, as predictable as a late-period Rolling Stones album and drained of surprise. Play these albums now, and they sound like calculated, inferior versions of the band's callow but ecstatic early-'80s releases.

Nonetheless, the albums served a broader commercial purpose. They reassured fans who had strayed from the band through its '90s experiments, and helped U2 fill arenas around the world. But the Irish quartet had lost something crucial in trying so desperately to connect with iPod nation. Everything sounded too pat; calculation had trumped inspiration.

"No Line on the Horizon," released Tuesday, tries to restore what's missing by pulling things apart and letting the songs breathe. The band has reconnected with its two most trusted collaborators, Brian Eno and Daniel Lanois, and given them an even greater role. They not only co-produce the album, but play some of the music and share in the songwriting.

The songs have become more amorphous; five stretch past five minutes.

The melodies reveal themselves more reluctantly, and the layers undergirding them teem with ambiguity.

At times it's uncertain who or what instrument is creating the sounds heard on several tracks; Larry Mullen shifts between live drumming and programmed beats, and keyboards mesh with guitars in a thick ambient mist. Only Adam Clayton's bass retains its singular personality; its prominence in the mix makes him the album's most valuable player, as he pushes the songs forward with a mixture of elegance and supple power.

The first three songs point a way forward. On the title track, melancholy synthesizers drift across the horizon like distant jets, playing counterpoint melodies over a thick stew of rhythm. Bono's voice sounds appealingly frayed, caught up in the whirl of sound around him. "Magnificent" lurks in the shadows for a minute, then breaks into a gallop over the Edge's ringing guitar and Clayton's bounding bass. It's a quintessential U2 moment: big and yet somehow vulnerable. "Moment of Surrender" completes the journey, and lays out one of the album's key themes: "Two souls too smart to be in the realm of certainty."

Nothing can be taken for granted. Anxiety hovers like the organ chords draped over Bono's patient vocal melody. Once again, Clayton's bass serves as an empathetic foil, standing just off the vocalist's shoulder, answering his every line.

At its best, "No Line" is about sonic drama. "Let me in the sound, meet me in the sound," Bono demands on at least two tracks. But in the middle of the album, U2 loses its nerve, and rapture gives way to formula. It's as if the band were hedging its bets, knowing it needed a couple of stadium-ready songs when it visits the world's sports arenas later this year.