One week before U2 boarded one of the last commercial flights into East Berlin to make the album that would become Achtung Baby, they unveiled a new song, a veritable harbinger of their music to come.

It arrived on the Aids-benefit album Red, Hot + Blue, which was released on September 25, 1990. It featured contemporary artists putting their unique slant on Cole Porter compositions and U2 offered an especially inventive reinterpretation of Night and Day.

It was the first track they had brought out in the 1990s and it was quite unlike anything they had done before. The sonic backdrop was eerie, edgy and evocative of the thrills and dangers of the nocturnal world. Clattering beats and elements of industrial rock offered an oddly pleasing counterpart to Bono’s croon.

The fingerprints of Mark Ellis — aka Flood — were all over it. The English producer had been an engineer on The Joshua Tree, but his gifts in the studio were especially apparent on Violator, Depeche Mode’s big-selling seventh album which had been released in March. Now, he was helping U2 reinvent themselves and much of June 1990 was spent getting Night and Day absolutely right.

The previous December 30, Bono told the crowd at Dublin’s Point Depot that U2 would be going away to “dream it all up again”. It seemed as though the band had become jaded with their hugeness and, one imagines, stung by some of the reviews that greeted both the Rattle and Hum album and movie.

U2 have spent much of their career talking up new projects, then often not quite delivering, but Achtung Baby would be a true reinvention.

The process began shortly after the Lovetown tour ended in early 1990 — and well away from the spotlight. Bono and the Edge had agreed to work on the soundtrack for a new London stage adaptation of Anthony Burgess’s most famous novel. A Clockwork Orange 2004, as it had been renamed, required an avant garde approach and, heavily inspired by uncompromising German industrial bands such as Einstürzende Neubauten and KMFDM, the pair quickly delivered music that was at odds with anything they had done before.

Poor reviews for the production meant a very short run and the contribution of Bono and the Edge passed under the radar. None of their compositions were released at the time although, years later, for the 20th anniversary deluxe version of Achtung Baby, one of the tracks, Alex Descends Into Hell for a Bottle of Milk/Korova 1, was finally unveiled.

It provides an intriguing snapshot into the direction U2 were taking — one where risk was central to their art. Both that experiment and Night and Day constitute the sound of a band throwing caution to the wind. It would bode well for the sessions in Berlin.

In fact, a handful of Achtung Baby’s key songs began life in Dublin’s STS studios in the summer of 1990, but they were still little more than fragments before shape was put on them in Hansa Studios.

The first thing the band did on arrival in Berlin was to hook up with the auteur filmmaker Wim Wenders and shoot a video for Night and Day. It’s an intriguing artefact: the music is a world away from what they were doing on The Joshua Tree and Rattle and Hum, but their look is still the same. There was some time to go yet before Bono’s bug-eyed shades and Adam Clayton’s peroxide hairdo.

Work began in earnest, although the pre-Christmas session at the studio next to a dismantled Berlin Wall were fraught with difficulty. Bono and the Edge were keen to push forward with new ideas and new sounds, while Clayton and Larry Mullen pondered what their role in the brave new U2 world would be.

Daniel Lanois was with them for those difficult months. He didn’t just work his alchemy in the studio but also put his strong people skills to good use, keeping the quartet buoyant when the new songs were struggling to coalesce.

It all began to click into place with One. The band have often cited the track as among their most important songs, and with good reason. It didn’t just provide Achtung Baby’s emotional heart and a key live track for every tour onwards — it kept the band together.

Brian Eno, who had worked with U2 since The Unforgettable Fire album, came to studio when the rudimentary work had been done on the album and his contribution was enormous. The former SS dancehall had a special place in his heart — it was there, in the late 1970s, that he had produced two of David Bowie’s most vital albums, Low and Heroes. They offered further revolution to rock’s great shapeshifter, and now he was helping to fashion the 1990s U2.

His emblematic sound is there right at the start, as Zoo Station unfurls with synthesised, disjointed beats over a sonic backdrop that is one-part ambient, one-part industrial rock.

Eno’s imprint also helps make The Fly the ultimate anti-U2 song. It was the first single released from the album in October 1991. It became only the second U2 single to top the UK chart (Desire was first), dislodging Bryan Adams’ 16-week stint at number one with (Everything I Do) I Do It for You.

As a taster for Achtung Baby, The Fly was the best possible choice. It was abrasive and thrilling, sonically, and Bono’s aphoristic lyrics were suggestive of a man who had shaken off his earnestness. The video, shot in London and Dublin, ushered in a whole new look — shades for the frontman (he has not been seen without some form of sunglasses ever since), lots of leather, a beanie for the Edge. It could hardly have looked less like the scowling foursome who appeared on the cover of The Joshua Tree. Bono, always one for the pithy observation, noted that The Fly was “the sound of four men chopping down The Joshua Tree” and he is absolutely right. He could have said the same thing about Achtung Baby in is entirety.

Thirty years after it was released, the album is considered by many to be U2’s crowning glory. There isn’t a weak track on it and although One and Mysterious Ways would likely be in anyone’s top 10 U2 songs, the lesser heralded Ultra Violet (Light My Way) and Love Is Blindness endure just as strongly. The former is a personal favourite.

The critical consensus at the time was that the band had made a masterpiece. In a perceptive review, Hot Press editor Niall Stokes wrote: “Achtung Baby plunges into the rich complexity of adult experience, the spiritual, the cerebral and the sensual all clashing in a cauldron of ambition, insecurity and desire. Ostensibly decadent, sensual and dark, it is a record of, and for, these times.”

That some of its most emotive songs were inspired by the Edge’s splintering marriage adds to the sense of a band delving deep into themselves in the name of art.

The U2 who made Achtung Baby were barely into their 30s. Their career was still comparatively young — their debut album Boy had been released just 11 years before — but what’s evident now is their desire to push themselves, to not settle with ‘good enough’.

It’s a very different U2 today. It’s been more than four years since their last album and there have only been faint noises about a new one. A new single, Your Song Saved My Life, which was recorded for the Sing 2 movie is U2-by-numbers, and perhaps the blandest single they’ve ever released.

It follows an equally dull anthem penned as Euro 2020’s official song. We Are the People is arguably more about collaborator Martin Garrix than U2, and while the brief for the football tournament wouldn’t have offered much in the way of creativity, it also suggested that the band were content to go through the motions.

Achtung Baby marked the beginning of U2’s most daring decade. They released Zooropa shortly afterwards — an off-the-cuff album just wouldn’t happen today — and sought further reinvention with 1997’s Pop. That album is now seen as something of failure and they played it safe on its follow-up, All That You Can’t Leave Behind in 2000.

There’s no debate over Achtung Baby’s merits — the album, ultimately, that even U2-haters have a soft spot for.

The top five: John Meagher’s favourite U2 albums

Achtung Baby (1991)
The word ‘masterpiece’ is bandied about a little too often in rock, but this is one ensured U2 could be every bit as big in the 90s as they were in the 80s.

The Unforgettable Fire (1985)
Their fourth is now seen as a ‘European’ album. Heart, soul and experimentalism.

The Joshua Tree (1987)
It was the album that made them “Rock’s hottest ticket”, to quote Time magazine, front-loaded with huge songs central to U2’s live shows ever since.

Boy (1980)
The sound of a young band going for broke. The Edge is the star, and Bono’s impassioned vocals leave a mark.

Zooropa (1993)
One of the band’s more playful and daring efforts.