At the dawn of the 1990s, most of the biggest bands in the world – Def Leppard, INXS, Bon Jovi, Mötley Crüe, Poison – marched blindly into the new decade, releasing a watereddown retread of their last album. Not U2. Abandoning the stadium-size sincerity of LPs like The Joshua Tree, they went to Berlin, drew on krautrock and club music and created what Bono called "probably the heaviest record we've ever made" – a barrage of irony, distortion and still-huge hooks.

This deluxe box set features both Achtung Baby and its spacedisco follow-up, 1993's Zooropa, along with B sides, remixes, previously unreleased outtakes and a Kindergarten disc, basically an early version of Achtung Baby. The best of the unheard tracks may be "Down All the Days," an early version of "Numb" with Bono on vocals instead of the Edge. Others are more interesting than thrilling, like "Oh Berlin," a soaring ode to the birthplace of Achtung Baby, and "Heaven and Hell," a moody synth number with some unexpected doo-wop flavor. A few tracks on Kindergarten sound nearly identical to the final versions; others went through major restructuring: In its early form, "One" sounds like a campfire singalong. The bonus material is not essential listening, but since U2 rarely pull back the curtain on their creative process, it's fascinating to hear this rough draft of history.

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• U2 Revisit 'Achtung Baby' and Question Their Future
• Photos: Three Decades of U2 Onstage and Backstage

“Ambling Alp”

The "Ambling Alp" was the nickname of the six-and-a-half-foot-tall Primo Carnera. Though the song is named after the Italian-born 1930s heavyweight champion, Yeasayer are actually paying tribute to boxing legend Joe Louis with this first-person psychedelic dance-rock tune. “I was always interested in writing a song that had boxing mythology in it,” Yeasayer’s Chris Keating said. “It’s pretty fascinating: There were so many amazing characters, and it was so closely entwined with 20th century history.” Yeaseyer also invokes German champ Max Schmeling and hints at the historical significance placed on the historic bouts between the Nazi-era boxer and the African-American Louis.