One rumor goes that Stevie Nicks threatened to leave Fleetwood Mac if they actually called the album Tusk. “There was nothing beautiful or elegant about the word ‘tusk,’” she later said. She’s right: It’s a grunt, a jab, a thudding monosyllable that has none of the musicality of the title Nicks was already dreaming up for her first solo record, Bella Donna. It was also, at least according to another rumor, a dick joke: “I don’t recall it being Mick’s joke about a …,” she trailed off in that interview, as if she couldn’t even bring herself to say it. “That went right over my little prudish head. I wasn’t even told that until after the record was done, and then I liked the title even less.”

Another way of thinking about the title, though, is as an outgrowth of the decorative, costly excess that birthed it: Before Fleetwood Mac even arrived at Studio D at L.A.’s Village Studios in 1978, all sorts of exotic knickknacks were imported onto the premises, transforming the space into a simulacra of an obscenely rich rock star’s home. In the liner notes to the album’s 2004 reissue, Nicks set the scene: “shrunken heads and leis and Polaroids and velvet pillows and saris and sitars and all kinds of wild and crazy instruments and tusks on the console.” Photographs by the nature artist Peter Beard were scattered around for inspiration. “Rare woods from Brazil and volcanic stones from Hawaii went into the decor,” Nicks’s biographer, Stephen Davis, writes. All this ambiance—and Lindsey Buckingham still insisted upon recording some of the damn album in a bathroom.

Such is, as Mick Fleetwood aptly puts it in his biography, “the duality of Tusk.” A sprawling double album, it’s rife with contradictions and ironies. It was, at the time of its release 40 years ago this week, the most expensive album ever made (and the first album ever to cost more than $1 million to record), but its rough edges and experimental ethos have since made it a source of inspiration within the indie-rock world. (In 2002, art-rockers Camper Van Beethoven released a great, imaginative full-album cover of Tusk.) An intentional departure from the coiled energy of Rumours, Tusk is a record large, strange, and varied enough to contain its exact opposite: It is at once sprawling and intimate, masculine and feminine, successful and failed.

It is also the rare album that could sell more than 4 million copies, spawn six hit singles, and, relatively speaking, still be considered a flop. “I say this without hesitation; as a band we really didn’t give a shit,” Fleetwood writes of the record’s commercial prospects. “Not at all.” Not many artists have to confront the mixed blessing of following up what was then the best-selling record of all time, and no one in Fleetwood Mac would have felt creatively satisfied had they just made Rumours II. Still, few listeners or record executives could have quite anticipated the strange sprawl of Tusk, a record that over the years has earned numerous comparisons to the Beatles’ White Album. (Fleetwood, in his memoir, refers to the group’s 1975 self-titled release as their “‘White’ Album”—but he means that in a different sense.) But time sands all edges. Forty years after its release, Tusk feels not so much like an anomaly as an archetype, the urtext of the Difficult Follow-Up Album, and—wild as this would have seemed at the time of its release—increasingly the consensus choice for Fleetwood Mac’s finest record.

The year before he entered the studio to begin recording Tusk, Lindsey Buckingham saw the Clash live in London. The experience left him electrified, challenged, and a little bit personally offended. In late-’70s Britain, writes Nicks’s biographer Davis, “Fleetwood Mac, the Rolling Stones, Led Zeppelin, Paul McCartney, Elton John, and all the older musicians were mocked for being out of touch with their audience and reviled as ‘dinosaur bands’ and Boring Old Farts.” Buckingham was then not even 30; he wasn’t about to live up to the Boring Old Fart stereotype. So he chopped off his flowing Led Zeppelin locks and traded in his bell-bottoms for skinny jeans and tailored suits. He started hearing beauty in dissonance, like the way recording on a boom box could give music a stark, compressed sound, or the tone you got from playing percussion not on a drum kit but an empty box of Kleenex.

An underappreciated aftershock of punk’s first wave is the kick in the ass it gave to some of the previous generations’ heroes, pushing some of those “dinosaur bands” to make their most adventurous music in years. Punk dared the Stones to make 1978’s Some Girls, their best and most brash record since Exile on Main St. It’s also the inspiration for some of the great Buckingham compositions on Tusk, from the taut, sneering “What Makes You Think You’re the One” to the haunting, oddly dissonant last-call dirge “That’s All for Everyone.” Buckingham was constantly experimenting in Studio D, searching for undiscovered tones and textures: He got the grumbling, blown-out sound of excitable punk ditty “The Ledge” by tuning his guitar down to sound like a bass. (“It sounds to me like it was put in a cement mixer and almost spat out,” he later said, proudly.) “I remember Lindsey used to make such a horrible sound,” the album’s coproducer, Ken Caillat, said in Ryan Reed’s book Fleetwood Mac FAQ. “He would physically make me distort the guitar so that it sounded like fingernails scraping across a chalkboard. I remember when he was recording ‘Not That Funny,’ he insisted he wanted a really weird-sounding vocal, so he made us tape a microphone to a tile floor, and he was doing a push-up over the microphone, singing, ’Not—that—funny—is it?!’ Anything to make it weirder was better on his songs.”

The Beach Boys, too, cast a long shadow over Tusk, and for several different reasons. Ever the studio rat, Buckingham spent the months leading up to the Tusk sessions listening obsessively to Pet Sounds, trying to deconstruct the production techniques behind that innovative masterpiece. It’s also been reported that Buckingham was granted access to the elusive master tapes of the Beach Boys’ then-unreleased Pet Sounds follow-up Smile, and that the Tusk tracks “That’s All for Everyone” and “Beautiful Child” bear the influence of Brian Wilson’s cutting-edge production. During the Tusk sessions, though, Christine McVie went even straighter to the source—she actually started dating a Beach Boy, the dreamy but ultimately troubled drummer Dennis Wilson.

As a counterbalance to Buckingham’s punk outbursts, Tusk showcases some of McVie’s most straight-forwardly lovely compositions: opener “Over & Over” sets a rose-colored tone, while the understated “Never Make Me Cry” is a perennial tear-jerker. Perhaps the most Rumours-reminiscent cut is McVie’s rousing “Think About Me”—one of Tusk’s few full-band jams. Tusk wouldn’t have confounded listeners if even half its songs sounded like this, but restless shape-shifting was also a consistent part of Fleetwood Mac’s ethos, even from the Peter Green days. “They had been a blues band, then a jam band, then a rock band, then a soft rock supernova,” Davis writes. “The Rumours groove had to be part of a progressive continuum, not the endgame.”

One of the most acrimonious fights during Rumours was over the exclusion of Nicks’s masterpiece “Silver Springs.” The band had to make some cuts to keep Rumours confined to a single LP, and when it came time for the final sequencing, the languorous, slow-tempo-ed “Springs” was first on the chopping block. “I started to scream bloody murder and probably said every horribly mean thing you could possibly say to another human being and walked back in the studio and completely flipped out,” Nicks said years later, recalling the conversation with Fleetwood when she first learned the song’s fate.

She got her revenge on Tusk. While Buckingham often approaches songwriting like a code to be cracked (“I’ve learned more about the mathematics of songwriting—how to fit pieces together, line length, timing chords and melodies,” he said around the time of Tusk’s release), Stevie’s process was more intuitive, her songs less rigorously structured. She thrived in open space and sprawl, something Tusk generously supplied. Her songs on the record are loose, unhurried, and exploratory, from the poignant ballad “Storm” to the meditative confessional “Beautiful Child.” The bluesy rocker “Angel” showcases a gravely, newly mature tone of Nicks’s voice that she’d explore further on Bella Donna, while the fan-favorite “Sisters of the Moon” furthered her witchy self-mythology: “A black widow spider makes / More sound than she,” Nicks sang, “and black moons in those eyes of hers / Made more sense to me.”

Related The Enduring Power of Stevie Nicks

Her most enduring offering, though, is “Sara”—more of an incantation than a pop song, though it was still Tusk’s highest-charting hit. Her first demo of the track was 16 minutes long; a gorgeous nine-minute take was included on Tusk’s 2004 reissue. In his biography Gold Dust Woman, Stephen Davis calls “Sara” “the most asked-about song by Stevie’s interviewers, even more than ‘Rhiannon.’” Their first question, of course, was almost always, “Who’s Sara?” but that was a misleadingly literal thing to ask of a Stevie Nicks song. “It’s not about Sara [Rector], who was one of my best friends—even though everybody thinks it is,” she said many years later. “But it was really about what was going on with all of us at the time … some songs are about a lot of things.” “Sara” is an impressionistic swirl of all that was happening in Stevie Nicks’s mind in the heady days of 1978, from her ill-fated affair with bandmate Mick Fleetwood to her indecision about whether or not to pursue a solo career. It was a blustery brew but, as she’d tell us a few songs later, she had always been a storm.

While she believed Tusk to be “a spectacular record,” soon-to-be solo star Nicks resented the time its recording required of her. “Tusk took us 13 months to make, which is ridiculous,” she said when promoting Bella Donna in 1981. “I was there in the studio every day—or almost every day—but I probably only worked for two months. The other 11 months I did nothing, and you start to lose it after a while if you’re inactive. You see, Lindsey, Chris, John, and Mick all play, and I don’t. So most of the time I’d be looking at them through the window in the control room. After four or five hours, they’d forget I was even there, they’d be so wrapped up in little details. It was very frustrating.”

One of the approximately 3 billion things I adore about Tusk is that it contains maybe the greatest, and definitely the most petty, album credit of all time:

I completely forgot the production credit on Tusk is “Fleetwood Mac (Special Thanks from the Band to Lindsey Buckingham)” !!!! I will love this petty messy band until I die pic.twitter.com/l3gCk0RciY — Lindsay Zoladz (@lindsayzoladz) October 21, 2018

Buckingham was, more than anyone, the sonic mastermind behind Tusk. But the very fabric of Tusk is also variety, collaboration, and bricolage—an alchemy he never could have achieved alone. If Rumours was the result of a handful of passionate, often-inebriated people standing elbow-to-elbow in a too-small room, Tusk is the sound of them stomping into their respective corners. To love Fleetwood Mac is to marvel at the beautiful absurdity that these five very different people were ever in a band together, let alone a band whose songs could hang together so well. In this sense, the improbably cohesive Tusk just might be their defining record.

Maybe it was just ahead of its time. Tusk’s double-album breadth might have stunted its commercial prospects in 1979—the 2XLP retailed for $16.98, around $50 adjusted for inflation—but in the more-is-more logic of the streaming era, it seems downright normal. (Drake’s mammoth-selling 2018 album Scorpion, for one example, is 15 minutes longer than Tusk.) Forty years later, it remains the blueprint for what comes after astounding commercial success, if an artist is too itchy and creative to simply rest on their laurels. Its forward-thinking ethos has kept it fresh all this time. “Tusk is not going to sound dated in five or 10 years,” the writer Blair Jackson predicted all the way back in 1981, “and I would be willing to bet that a lot more people will slowly be convinced of the album’s greatness than will forget all about it.” You can say that again.