With their second album, Ágætis byrjun, Sigur Rós knew only that they wanted to make things bigger. Their first record, 1997’s Von, was dark and, by the standards of what they became famous for, positively screechy: Back then, they were inspired by the hurtling propulsion of Smashing Pumpkins and My Bloody Valentine, bands that generated soothing textures from cacophony. Von sold 300 copies in Iceland. But the dismal showing left no seeming dent on young Jónsi Birgisson’s confidence. The singer posted a salvo on the band’s website prior to Agaetis’ release: “We are simply gonna change music forever, and the way people think about music.”

It’s alarming to consider, from the vantage of 2019, the degree to which he seems to have accomplished his mission. If we now live in a world of small, soft drones, a pruned garden of “Lush Lofi” and “Ambient Chill” and “Ethereal Vibes” Spotify playlists, we can blame this condition, at least in part, on the impact of Ágætis byrjun. It is an album that has terraformed our landscape—so much of our lives now sounds like it, from Nissan commercials to “Planet Earth” documentaries to the long trail of ads that could not procure Sigur Rós’ approval and went about constructing benign replicas of Sigur Rós songs instead.

Before Ágætis, post-rock was a niche concern, a tiny sub-sub-genre centered around a dozen or so bands in England and North America—Stereolab, Bark Psychosis and a few others in London; Tortoise and Gastr del Sol in Chicago; Godspeed You! Black Emperor in Montreal. After Ágætis, the sound—massive, surging, triumphal; melancholic and soothing and mostly major-key; wreathed in strings and horns and ripe with melodrama and headlocking you into transcendence—is a global phenomenon. They opened for Radiohead; they turned down a slot on “Letterman” because the host wouldn’t give them enough time. They even appeared on “The Simpsons.” Twenty years into their career, they tour arenas and command a massive following. They are a cultural institution.

It’s hard to know if Ágætis byrjun catalyzed the massive shifts that unfolded in its wake, or if those shifts were already brewing, in search of a seaworthy vessel to carry us wherever it was going. Today, Sigur Rós’ career seems like a natural and desirable trajectory: Get your music into the ears of some important people (in Sigur Rós’ case, it was celebrities like Brad Pitt and Gwyneth Paltrow); from there, your music might shoot outward into some large-scale and modestly experimental commercial film (Tom Cruise and Cameron Crowe’s Vanilla Sky); and then it can rain down into dozens and dozens of television shows via the diligent work of music supervisors. But when it all happened to Sigur Rós, it was all pretty new, and it was all happening to the music industry at the same time.

To make the album itself, they recruited a keyboardist named Kjartan Sveinsson, who knew a lot more than they did about the things they were interested in—arrangements, composition, songs that sounded like cavernous day spas. They enlisted producer Ken Thomas, who started out as an assistant working on Queen albums before moving on to early industrial acts like Throbbing Gristle and Einstürzende Neubauten. He also mixed the first record by Björk’s early band the Sugarcubes, which is what led him to Sigur Rós.

With Thomas, they built a record that felt like being stuck inside a church bell. Their enormous sound came not from size, but from scale. The distance between the quietest noises—the little cymbals ticking the eight notes on “Svefn-g-englar,” Birgisson’s falsetto—and the loudest ones—say, the drums and organ that land like Thor’s hammer about six minutes into the same track—feels measurable only in miles. It is a long, liquid sound, devoid of sharp points: Even the most massive dynamic shifts happen with rounded edges. The drums are nested inside so much reverb that you can nearly hear the air gathering around the snare head before impact. Birgisson played his electric guitar with a cello bow, which offered the sonorous tones of feedback without the disturbance of picks. It is thunderous and dreamy, soothing and stirring—a big, frosted wedding cake of mallet percussion and pianos and strings and piping, cooing vocals. It is a sound designed to overwhelm, and it does, which is probably how British critics ended up gasping that the music was “like God weeping tears of gold in heaven.” Music of this scale is never kind on the higher faculties.

The album is a triumph, above all, of arrangement and engineering. When the piano kicks in on “Starálfur” (the same one that accompanies the discovery of the mythical jaguar shark in The Life Aquatic with Steve Zissou), I still have to suppress a delighted giggle of wonder. It’s like watching an invasion of CGI superheroes, or (I imagine) revving a high-performance car and watching the speedometer float. It isn’t so much a sound as a special effect, and it communicates with your brain solely in dopamine floods.

If you are inclined to sniff suspiciously around grandiose music, examining it for kitsch, you probably reeled away staggering from Sigur Rós, who proudly stink of it. This was another part of their appeal and their strength: The music is texturally complex, for sure, but the emotional framework is deliberately simple and clear. They are gloriously unafraid of blast-off. The piping melody that ends “Olsen Olsen,” doubled up with horns and a choir, is straight out of a Mannheim Steamroller Christmas album.

Live, they maintained this communal feeling without sacrificing clarity. You can hear this in the live recording included in a generous and fulsome new 20th-anniversary reissue. The gig was June 12, 1999 at Reykjavík’s Íslenska Óperan—an album-release celebration. They were brand new to this material, but somehow they sounded as commanding then as they do now. The box set also includes reams of demos and half-finished versions of Ágætis byrjun—they provide a nice glimpse into the band’s working method, which was open-ended and involved multiple versions of the same song, some with or without vocals or at different speeds. Spending time with all of these raw tracks is a bit like opening up a “version history” in Google Docs—you learn a little bit about how the final product came to be, but it only serves to heighten your appreciation that you were spared the editing process.

Parsing the re-release, I was drawn back to the album itself again. It doesn’t really require elaboration, or added context. It’s entire appeal lay in the sense that it dropped, immaculate and mysterious, from the sky. Unless you were Icelandic, you didn’t know what they were saying—and often not even then. On Ágætis, Birgisson famously dabbled in an invented language called Hopelandic—some on “Olsen Olsen,” and some lightly sprinkled throughout. This might have spurred some listeners on to discover “what he was saying,” but for most of us, he was saying whatever we heard. His words were not messages, they were bird calls. The single most indelible word Birgisson has ever sung—“tju”—is a gibberish syllable, a refrain from “Svefn-g-nglar” that sounded then and will always sound just like “It’s you.” There were no other meanings within to parse or contemplate—just a pretty sound. We heard ourselves in it.