U2’s most transformative album—the one that would rejigger the band’s sound, re-energize its members, and win over even the band’s fiercest critics—began with a vague promise from Bono. On December 30, 1989, during a homecoming performance in Ireland, the U2 frontman bid farewell to the ’80s by declaring that his world-conquering band was taking a sabbatical from stardom. “We have to go away,” Bono announced, “and just dream it all up again.” The hasty retreat made sense, given the response to 1988’s highly hyped Rattle and Hum, an ambitious double album (and documentary film) that found U2 traveling through America in search of new sounds—and, to some observers, wallowing in self-seriousness. The band had always prompted its fair share of eye-rolling, but Rattle and Hum gave the group’s detractors even more ammo: Gospel choirs? Folk songs? Just who do these U2 guys think they are? The band members were wondering the same thing. So in 1990, they decamped to Germany, hoping the recent fall of the Berlin Wall could be the backdrop for a future-focused burst of creativity. Instead, the four band members—Bono, guitarist the Edge, drummer Larry Mullen Jr., and bassist Adam Clayton—began sparring in the studio, torn over what the ’90s version of U2 should sound like. The Edge, in particular, was fascinated by the noisy new wave of industrial artists like Nine Inch Nails and KMFDM—acts whose menace and aggression were far removed from U2’s brand of guitar-starred anthems. Not everyone was sold on the idea of a more aggro, less earnest version of U2. But the push and pull of the Achtung Baby sessions—which were later continued in Dublin—would result in 12 tracks of buzzsaw rock that sound as though they’d been created by a different band altogether. The album’s lead single, “The Fly,” is a grimy dance-floor come-on anchored by Clayton and Mullen’s strange new rhythms. And on “Mysterious Ways,” the Edge trades in his familiar clean-lined guitar-jangle for a hazy funk that sounds like it’s being played through a dial-up modem. As for Bono: He’s never been as loose as he is here—nor as libidinous. On both the kaleidoscopic “Until the End of the World'' and the clanging “Zoo Station,” he adopts a winking, playfully skeevy persona—the perfect guise for the ironic ’90s. Yet the clear-eyed bluntness that propelled U2 to infamy in the ’80s was still intact, most notably on “One,” a soulful ode to reconciliation that would become one of the biggest songs of U2’s career. Less than two years after U2 vowed to “dream it all up again,” Achtung Baby delivered on that big promise—maintaining all the vigor and fervor of the band’s early years, while flirting with sounds that would soon define radio: trip-hop, shoegaze, electro-pop. This album wasn’t Rattle and Hum—it was pure Sturm und Drang. And for once, the group’s most loyal fans—as well as its most exhausted critics—could agree: This souped-up, super-improved U2 2.0 was even better than the real thing.