The only thing more remarkable than Sigur Rós’ success is how remarkably surprising it is.

Sigur Rós rose to prominence in the turn-of-the-millennium post-rock boom, alongside such bands as Tortoise, Mogwai, Godspeed You! Black Emperor, and Explosions in the Sky. Post-rock presents distinct listening challenges: It’s largely based on rock, but it doesn’t overtly rock, per se, rejecting much of the genre’s traditional elements — melodic hooks, simple song structures, three-minute song lengths, etc. — for parabolic epics built on a majestic churn between peak and valley.

Yet while Godspeed You! Black Emperor’s apocalyptic suites and Explosions in the Sky’s widescreen instrumental narratives come with certain challenges, Sigur Rós’ beatific art-rock came with even more: Sigur Rós’ catalog is characterized by flowing, amorphous puddles of variegated tones, predicated largely on the sound of gauzy synthesizers and violin bows scraping across guitar strings. It’s at times an incredible passive listening experience, sweeping the listeners up in drifting buildup and inevitable crescendos. It’s the kind of music that elicits hyperbole from rock journalists; one British critic, writing in Melody Maker, wrote that the band’s landmark 1999 record Ágætis Byrjun sounded like “God crying golden tears in heaven.”

Further, singer Jón “Jónsi” Þór Birgisson coos in his native Icelandic — or, at least on the band’s first few records, in a nonsense language preciously called Hopelandic. And hell, it’s hard enough to correctly pronounce the band’s name, let alone its heavily accented song titles. (It’s SIH-ur ROHSH, by the way; the band’s website has a handy pronunciation guide.)

It’s not exactly a recipe for arena-level success.

And yet Sigur Rós has reached heights heretofore unvisited on its soundtrack rock contemporaries. Where many of those bands subsist by scraping together club-show tours, Sigur Rós packs arenas: In March, the band sold out Madison Square Garden. Jónsi and company also bounced around the major-label circuit for a while, too, releasing 2002’s () on MCA and 2005’s Takk on Geffen. The band’s latest, Kveikur, was released in June via major-indie XL. Sigur Rós has also opened for Radiohead, worked with a 90-piece orchestra, and had its music featured in several films. The Icelandic outfit even scored an episode of The Simpsons, and right now Jónsi and the boys are currently in Croatia filming an appearance for the upcoming season of Game of Thrones.

Just how did this little band from Iceland, which is now nearing its second decade, connect with an audience that, by and large, doesn’t even speak its native tongue?

“I don’t know,” bassist Georg Hólm laughs. “We get asked a lot about the meaning of the songs and Icelandic and all that. The lyrics usually are just what we feel. It’s what we feel the song is about. So I think that our music has this emotional element, and the music speaks for itself. And I think that’s what people connect with. It’s not a simple four-chord song.”

Hólm adds, “It’s the old cliché, music is the universal language. But it’s true. Music can actually talk to you. And we try to do the best we can to make it talk.”

Kveikur, Sigur Rós’ ninth long-player, roughly translates in English to “wick” or “fuse.” It’s appropriate. The opening track “Brennisteinn” begins with a 20-second blast of white noise, a sizzle that sounds like a lit fuse, before exploding into a conflagration of fuzz bass and feedback. It’s an opening that would normally be more at home on an Isis record than a Sigur Rós one, and though Jónsi’s familiarly glossy, spectral vocals come in quickly, it sets a tone: Kveikur is darker, moodier, and far more aggressive than previous Sigur Rós records.

Indeed, Kveikur is not so much a radical reinvention as it is a great reset: Keyboardist Kjartan Sveinsson, the guy with formal training who’d arranged their strings and played much of the non-rock instrumentation that made Sigur Rós’ orchestral splendor possible, left the band after last year’s Valteri. The hole Sveinsson that left behind clearly challenged the remaining three band members to fill it with something besides keyboards.

“I’m not saying it’s because [Sveinsson] left, but it’s one of the most fun records we’ve made,” Hólm says. “That’s not to slag him off, but there was a change in dynamics. We were really free. We just kind of did whatever. It was like, well, now we’re a three-piece. Let’s just start playing and see what happens.”

The band still sounds unsettlingly grand, and Jónsi’s ghastly wails are still lovely and haunting, but Kveikur finds Sigur Rós augmenting its panoramas with jarring dubstep beats (with the most success on the icy “Ísjaki”) and heavy metal textures and feedback (the lunging “Hraftntinna”). Hólm and drummer Orri Páll Dýrason let loose with lumbering bass bellows and pounding drum fills that rumble like subterranean waves. Kveikur is a primal and propulsive record, and it’s the kind of palette shift that shows just how versatile and creative the band can be.

“It was good for us to change the dynamics in the band a little bit,” Hólm says. “It was a lot easier that we originally thought, but it was good.”

As a result, Hólm’s more excited about touring behind Kveikur than he’s been since 1999’s Ágætis Byrjun. “We knew we wanted to do something different, something drastic, something radical for us. We knew we wanted to be darker, but we didn’t know in what way. We didn’t know it was going to be this much more aggressive and straightforward,” Hólm says. “That was a surprise, and a good surprise for us.”