KRUNK Their modesty was flattering, even if post-rock fans with functioning ears didn’t agree with the sentiment: Iceland’s Sigur Rós titled their breakout second LP Ágætis byrjun (“A Good Beginning” ), a charming nod to their humble origins in Reykjavík. But this sleeper 1999 LP—with its celestial bowed guitars and orchestrations, Georg Holm’s iceberg-thick basslines and the divine falsetto of frontman Jónsi—marked a Big Bang-level leap for experimental music in the mainstream. There hasn’t been a band that has replicated its tidal wave of emotions since. As Sigur Rós’ international profile has swelled, these songs have become soundtrack staples: It’s hard not to conjure the lucid dream madness of Vanilla Sky during the glacially paced centerpiece “Svefn-genglar,” an open-floodgates surge of feedback, haunted-house organ and lullaby coo; and the sonar ping piano and quivering strings of “Starálfur” feel inseparable from the tear-jerking jaguar shark scene in The Life Aquatic With Steve Zissou . Which is to say: Ágætis byrjun could be the most cinematic rock album of the last two decades. This massive 20th-anniversary edition, with its raw demos and alternate takes, opens a curtain behind that polished perfection—presenting the musicians not as the alien angels of the album cover, but as humans who occasionally played wrong notes like the rest of us mortals. Some of these extras are unnecessary, like an instrumental version of the shadowy “Ný batterí”—cutting Jónsi’s voice is like removing pepperoni from pizza for the fun of it. But the previously unreleased cuts (the bluesy, Krautrock-leaning “Hugmynd 2” and the chilling, dual-falsetto live duet “Debata Mandire”) present new sonic mysteries to savor.