How U2's technology is changing concerts for the better
"The two things you can't have up here are vertigo and a need for stability."
U2 bassist Adam Clayton is giving me a tour of "the barricage", a 29-metre long, double-sided LCD screen that the band climb inside of every night of their Experience and Innocence tour.
The walkway spans the entire length of Amsterdam's Ziggo Dome, linking the main stage and a smaller, circular platform at the far end of the arena.
"It's a very expensive way of getting from the big stage to this little one down here," laughs Clayton.
But it's much more than that. The screens rise and fall throughout the show, transmitting live footage, political slogans and colourful animations.
They can also go transparent at the flick of a switch, allowing U2 to appear and disappear behind the wall and even tussle with the images around them.
"The magic act, if there is one, is to shrink the venue and make it disappear," says Bono, who barrels around the 89-ton structure every night with a perilous disregard for his own safety.
"When you get an intimacy with a crowd of 20,000, it's a very moving thing."
U2 have been pushing the boundaries of concert technology since 1992's Zoo TV tour, an all-out blitz on the senses that combined 32 video screens, live satellite feeds and floating Trabants in a carnival of glamour, sleaze and irony.
The subsequent PopMart tour famously incorporated a giant revolving lemon, while 2009's 360° tour featured a $30m (£23m) custom-built steel "claw". But the aim has always been the same.
"When we were playing club gigs up and down the M1, it was always 'What's the fastest route to proximity with our audience?'" explains Bono.
"Now we use a lot of technology to serve that end. But it's the same thought: 'Is there a place in this show where people have a bad seat? Well, we're going to camp right there.'"
All of U2's stages are designed by the band's creative director Willie Williams in conjunction with London firm Stufish Entertainment Architects, whose other clients include Madonna, Lady Gaga, Take That and Beyonce.
There's "definitely an arms race of one-upmanship" between bands, says the studio's CEO Ray Winkler.
"Bands are very conscious that there's competition out there - so they aspire to deliver something bigger, brighter and better."
That's why you'll see Coldplay handing out wristbands that light up in time to the music, or Beyonce and Jay-Z floating over fans' heads on a hydraulic platform.
Winkler always asks his clients the same questions: "What is the story that you want to tell? What is the emotive arc of the show? And then, what are the means we can use to deliver that?
"Just because you have the technology, it doesn't mean you need to use it," he adds. "You have to use it intelligently."
U2 worked closely on their stage concept with Es Devlin, the British designer whose long list of credits includes Kanye West, Adele, and the opening ceremony of the 2016 Rio Olympics.
Together, they found a way of using the video screens that can be both impressive and disorientating.
In opening number The Blackout, silhouettes of the band flicker in and out of static, like ghosts trying to break into the realm of the living.
Then the musicians are revealed, inside the cage and thrashing about in strobe-lit angst.
But the show is just as powerful when the screens ascend to the ceiling and leave the band to perform Pride (In the Name of Love) on individual podiums situated at the four corners of the arena.
Do they ever miss the intimacy of playing club gigs?
"They were great when we were learning our jobs but it's very hard to go back there," says Clayton.
"By the time you get into your 30s, you want to be in theatres and arenas. You don't want to be going back."
So massive venues and big screens it is. But cutting-edge technology comes at a cost.
Across the music industry, average ticket prices are at a record high of $96 (£73) - up 14% from last year.
U2's tickets sit on both sides of that figure, reaching the eye-wateringly high price of £400 but bottoming out at just £40.
"We like to think our audience are not paying the price - it's us," says Bono.
"We could do a stripped-down show in a stadium, playing our greatest hits and the economics of that would be wildly more fair to the band. But I think it'd be unfair to the audience.
"We're trying to give them the best experience we can and we're prepared to pay for it… largely."
Putting their money where their mouths are, the band have revamped the entire Experience tour for its European outing.
It now opens with images of the continent in ruins after World War Two and ends with the band performing in front of the EU flag, making a plea for tolerance and unity.
"Starting the show after the Second World War, in that devastation, in that awfulness was a bit scary for us," says Bono.
"We opened the tour in Berlin and it was just silence, for very understandable reasons.
"But out of the rubble came this idea of Europe and that maybe, even though we speak different tongues, we could somehow use the same voice.
"I think it's a beautiful, romantic concept, Europe, but it's ended up as a cold concept for a lot of people. You just think about the bureaucracy in Brussels, which is why I guess a lot of people in the UK wanted to leave."
Have the band thought about how the pro-EU message will go down in the UK? "It's certainly going to be an interesting reception," says Clayton.
"I think our audience is probably going to be split several different ways, and I guess what we want to see is what comes out of it."
On a personal level, U2 are particularly worried about the Irish border situation.
"Since the peace process and that border evaporating, it's been a very different world for us," says Clayton.
"The closer we got to Europe, the more faint that border became. So we're very concerned about what happens between us and the North."
They don't pretend to have any solutions, even though Bono shoots off to Brussels after our conversation to rattle some cages.
Instead the repeated theme of their show is that love can be a force for change, redemption and resistance - even if you're part of a globe-conquering rock band.
"I think part of the feeling in a U2 show is that the four of us stuck together," says Bono.
On stage, he recalls how they were recording Achtung Baby in Berlin when the wall came down, but the band members were erecting walls of their own.
"It was important to tell people it has not always been rosy in the garden, as my dad would say. Achtung Baby was a very difficult time for our band. We nearly broke up before we got that song, One.
"So there are stresses and strains in trying to do your best work, as we still do. Every night it has to be the best night of anyone's life. And do you know what? That's a bit much."
That stress came to the fore last month when Bono's voice disappeared during a concert in Berlin.
The band were forced to abandon the show after just seven songs - and the singer was visibly emotional as he croaked out an apology to the audience.
"I didn't even have a sore throat before the concert," he says. "I was completely in perfect voice and then [snaps fingers] it was gone."
It's not the first time either. A case of "desert throat" affected the PopMart tour in Las Vegas, and in U2's autobiography Bono revealed he'd been diagnosed with "nasal allergies".
So is that what happened in Berlin?
"That's probably closest to the answer, actually," he says. "It's very humbling to be a person who can develop intense allergic reactions. That is not how I see myself."
Taking a break
The fact it happened in front of almost 30,000 people made it worse, he adds.
"Something like 5,000 of them had come from outside Germany, so they're not getting the cost of their flights back or their hotels back. I feel really awful about that, so we'll find a way to thank them."
Health-wise the singer has not had a great run over the last decade. He herniated a disc on the 360° tour and suffered multiple fractures in a "freak" bicycle accident in 2015.
In the sleeve notes for the band's latest album, Songs of Experience, he also refers to a recent "brush with mortality" that inspired several of his lyrics.
"I did have some issues," he says. "But they're gone and I'm not."
U2, however, are preparing a disappearing act of their own. As their Amsterdam show draws to a close, Bono thanks the audience and declares: "We don't know when we'll see you again."
Given the setlist leaned so heavily on their mid-career pivot into art rock, are they contemplating a similar period of reinvention?
Not necessarily, says Adam Clayton. "We've been hard at it for four years now - just constant touring," he smiles.
"I think we just need to go away for a little bit and give the audience a break."