Some numbers on the making of Led Zeppelin II. Studios: 13. Cities: five. Countries: three. Months between the band’s debut and the follow-up in question: nine. Number of those months spent on tour: seven. Days off? Zero—they found a studio. According to engineer Eddie Kramer, some of Jimmy Page’s guitar solos were recorded in hallways. In the interim, they dragged the album’s tapes between continents in a steamer trunk that got heavier at every stop. In other words, that Led Zeppelin II came to exist at all was a feat; that it was more radical, precise, dynamic, and fully realized than Led Zeppelin was—well, miracles are divine, and Led Zeppelin II was nothing if not the sound of earthbound hard work.

What had sometimes felt blocky or conceptual the first time around—British blues rock rendered slower, heavier, louder—now felt seamless, the sound of four players finding fluency in a new kind of language. Time on the road showed: A couple of the tracks here—“The Lemon Song” (adapted from Howlin’ Wolf’s “Killing Floor”), the John Bonham drum spotlight “Moby Dick”—either originated or evolved live, while others (especially “Whole Lotta Love”) reflected a kind of ecosystemic relationship between the players that made the music much more direct. This also enabled them to take bigger, weirder chances: the frenzied breaks that punctuate the “The Lemon Song,” for example, or the way “Ramble On” moves from hearthside folk to bruising rock with an almost iridescent continuity. Or—of course—how “Whole Lotta Love” snaps from its noodly, avant-garde middle section to its borderline-pornographic guitar solo. (Parents beware: These boys are sensitive and nasty.)

And while much has been made of the band’s liberal quotation of black American blues (including a 1985 copyright suit that dogged “Whole Lotta Love”), the reality—and legacy—was more complicated. Listen to Led Zeppelin II and you hear foreigners—young British men—absorbing blues not as a progressive pose but arcane knowledge, as gnarled and misty as the Celtic touches of “Thank You” or the Tolkien-inspired visions Jimmy Page leveraged into “Ramble On.” Put another way, Led Zeppelin II marked the moment that the band figured out how to wield blues as the sound of both this world and the world of a distant beyond.

So while other luminaries of the counterculture gathered for Woodstock, Led Zeppelin played San Antonio; Wallingford, Connecticut; and Asbury Park, New Jersey, where they were described by the Asbury Park Press as “an interesting, hard-working quartet.” (The move, according to the band’s manager, Peter Grant, was calculated: Led Zeppelin wasn’t part of a movement, and didn’t belong under a broader cultural banner. Fair enough.) At the end of the month, Black Sabbath played their first show and gave heavy metal its modern grimace, all four Beatles sat in the studio together for the last time, and Led Zeppelin went to New York to mix Led Zeppelin II. Two months later, “Whole Lotta Love” was released as a single, which probably would have been a nice occasion to celebrate had the band not already started working on Led Zeppelin III. You know what they say about rolling stones.