“A good start” is not quite right. That’s an accurate translation of the title Ágætis byrjun, and yet in precise terms it does not fit the album it belongs to. The LP that broke Sigur Rós and blew millions of minds in the process was not technically the band’s beginning. Their debut, Von, was released into obscurity in 1997 by Smekkleysa (translation: Bad Taste), the Icelandic label previously best known for launching the Sugarcubes. As the story goes, Von only sold 313 copies in its first year on sale. Ágætis byrjun did not immediately reach a large audience either when Smekkleysa released it 20 years ago today. But soon enough it would be inciting rapture around the world.

That’s the other reason Ágætis byrjun is a misnomer: “Good” doesn’t begin to do it justice. “Great” would also be selling it short, as would “awesome,” “excellent,” “terrific,” and just about any other pedestrian expression of positivity. Ágætis byrjun is worthy of every grandiose adjective you could throw at it. It’s one of those records for which overwrought praise feels entirely appropriate — not just an undeniable masterpiece but, for many, a religious experience that inspired evangelistic fervor. In the months and years after its initial drop, the album set off a global chain reaction that yielded releases in the UK in 2000 and North America in 2001, turning Sigur Rós into legends before they even released the follow-up in 2002. It was self-evidently brilliant at the time, and two decades later it stands as one of the most astounding and singular collections of music ever released.

In truth Ágætis byrjun emerged out of Reykjavík, but in essence it came out of nowhere. Even the few who encountered Von’s unremarkable longform experiments could not have anticipated this album; it would have scanned as extraterrestrial even without that illustration of an alien baby on the cover. Sigur Rós shared a mystical sensibility with Björk, their fellow Icelandic export. There were traces of Radiohead in the music’s astonishing ambition, and especially in Jón Þór Birgisson’s celestial falsetto. The mammoth post-rock of Mogwai and Godspeed You! Black Emperor, the dream-pop adventures of Slowdive and Cocteau Twins, the noise-blasted beauty of Sonic Youth and My Bloody Valentine — those references and more were there to be mined out. Yet the end result truly sounded like nothing else. It was like those UFOs Thom Yorke wished for on OK Computer had received his transmission and initiated contact.

For all its alien qualities, Ágætis byrjun was tapping into deeply human emotions, feelings so fundamental that it takes something startlingly intimate to arouse them. C.S. Lewis famously wrote, “If I find in myself a desire which no experience in this world can satisfy, the most probable explanation is that I was made for another world.” Lewis, a Christian theologian, was referring to heaven. But with his Chronicles Of Narnia, he became one of many creatives to map out a fantastical realm as an expression of that longing for the unknown divine. Countless books, movies, and video games have constructed universes like these, complete with their own geographies and taxonomies and mythologies. A few musicians have tried it, too, but not many of them have captured that breathless sensation and larger-than-life scope like Ágætis byrjun.

Ágætis byrjun is basically a movie unto itself, so it goes without saying that Hollywood music supervisors ate this stuff up. Cameron Crowe put “Svefn-g-englar” in Vanilla Sky, a film that ended with a spellbinding Sigur Rós live recording that would become ( ) centerpiece “Njósnavélin.” The particular beauty of “Starálfur” is so obviously cinematic that it was begging to be used in a stunning sequence like the underwater encounter in Wes Anderson’s The Life Aquatic. Still, I’m not sure any big-screen imagery compares to the visions conjured in my mind when I first played Ágætis byrjun on my friend’s stereo on a summer night, paralyzed with awe that music like this could even exist.

Sigur Rós had spent half a decade crafting a sound that seemed to exist outside of time, somehow geological and biological all at once, the personal blown out to universal proportions. Like the wardrobe in the Narnia books, Ágætis byrjun became a portal into that parallel dimension. To press play is to commence a journey, one you shouldn’t embark upon lightly. The album’s chasms are so vast, its climaxes so intense, that if you don’t give yourself over completely its weight can feel oppressive — not unlike the sensory deluge that has Jónsi shutting down like an overloaded android on “Hjartað hamast (bamm bamm bamm).”

There is no way to casually breeze through the glacial catharsis of “Svefn-g-englar,” a 10-minute song about sex that expertly mimics its ecstatic buildup and comedown, or the foggy wilderness of “Ný batterí,” a story of being released from captivity only to find yourself wounded and alone again. The album is a lot, and it moves at its own pace. Give it your divided attention and risk being lulled to sleep by long atmospheric passages or actively annoyed by Jónsi’s high-pitched moans. Surrender to it and you’ll find yourself soaring with “Viðrar vel til loftárása,” frolicking through “Olsen Olsen,” and settling softly into the tender embrace of “Ágætis byrjun” until you become one with the music.

The effect is perhaps even more potent for those of us who don’t understand Icelandic, who are freed up to project our own real or imagined drama onto these epics. In some cases we’re probably better off not knowing, such as “Flugufrelsarinn,” where Jónsi passionately emotes about saving flies by a lake. Even “Starálfur,” a song that feels plucked directly from a fairy tale, arguably makes a more profound impact when you don’t realize he’s singing about putting on his blue nightie and being visited by an elf. Perhaps Jónsi understood that Sigur Rós are at their best when the vulnerable and the unintelligible are colliding because by the next album he had gone all-in on Vonlenska (aka Hopelandic), a gibberish language he debuted on Ágætis byrjun’s joyous “Olsen Olsen.”

That next album, 2002’s ( ), would be the band’s second instant classic — a sequel that pushed out into darker, scarier corners of the Sigur Rós universe, with even lengthier swoons and more harrowing bombast. Together the two records represent this group at the peak of their powers, a pair of monumental works that soothe and throttle and thrill like nothing else. Sigur Rós had more great music ahead of them, but never quite this great. So it’s mildly insane to look up the lyrics to Ágætis byrjun’s lullaby of a title track and discover it’s about a band finding its footing but still unsatisfied with its progress. “We’ll do better next time,” Jónsi coos as the album winds to a close, “but this is a good start.” Even their understatements were titanic back then.

Sigur Rós are releasing a deluxe reissue of Ágætis byrjun on 6/21 via Krunk Records. Get details here.