To date, Bob Dylan's ongoing Bootleg Series has launched more than 40 discs worth of previously unreleased material at fans, spanning wobbly home recordings made when Dylan was still a Minnesota teen named Robert Zimmerman to “lost” tracks from this century. Last year's Cutting Edge set, in its deluxe iteration, offered a whopping 18 discs worth of studio sessions from Dylan's halcyon 1965-66 "electric" period: 20+ hours of false starts, fuck-ups and, of course, plenty of inspired outtakes of iconic songs. But that's just the beginning. In recent years, obsessives have also had to contend with enormous multi-disc copyright extension collections of live and studio recordings from the 1960s, which Sony has released in limited quantities in an effort to battle public domain laws in Europe.

But are the Dylan obsessives sated? Not likely. You can easily go down an unreleased Dylan rabbit hole via YouTube, where innumerable live shows, demos, studio outtakes, rehearsals, television appearances, and more reside for the intrepid Dylanologist to discover. (Tip: Searching the Dylan aliases “Elston Gunn” or “Blind Boy Grunt” will usually help you find what you're looking for.) There's more than enough unofficial archival material out there to keep the official Bootleg Series rolling for years to come.

But what about the stuff that's never made it out into the wilds, either officially or unofficially? In fan circles there are whispers of material that remains unheard or unseen by all but a few, existing either in Dylan's own vaults or in the hands of so-called "super collectors." Or perhaps some of it doesn't exist at all. But until every last second of Dylan's lengthy career has been dug up, the rumors will continue to fly. Ahead of Dylan’s 75th birthday on Tuesday (May 24), here are a few of the most enticing holy grails of Dylanology.

D.A. Pennebaker's Something Is Happening

Documentarian D.A. Pennebaker filmed Dylan's confrontational 1966 world tour, capturing the songwriter's drug-fueled madness both onstage and off in raw, glorious color. At Dylan's request, Pennebaker put together the 45-minute Something Is Happening, possibly to be aired on ABC. But Dylan didn't like the results, commandeered the footage, and made the infamous, rarely seen Eat the Document, which—as its title implies—is a fragmented deconstruction that definitely wasn't going to be aired on ABC. A few select journalists have seen (and raved about) Pennebaker's original cut (parts of which were used in Martin Scorsese's No Direction Home), but it remains frustratingly unavailable, even on bootleg. We’ll just leave Eat the Document below, for glimpses of what could have been.

The Complete John Wesley Harding Sessions

The masterful John Wesley Harding LP was recorded (and written, if Dylan is to be believed) in a matter of days in late 1967. It stands virtually alone among the songwriter's albums in that collectors have never been able to get ahold of any session outtakes or alternates. But they exist—and someone's heard them. "I heard a couple of alternate takes of 'All Along The Watchtower' that were, to me as a fan, just incredible," Michael Chaiken, the curator of the recently announced Dylan archive in Tulsa, teased Rolling Stone. Do these alternate "Watchtower"s include previously unknown lyrics? A distorted rave-up? A salsa-inflected arrangement? Time will tell... hopefully.

Bob Dylan & Crazy Horse

Neil Young and Dylan have maintained a mutual admiration society for decades now. But aside from a handful of live performances (see below, if you’re curious), they haven’t collaborated musically much. But according to Young biographer Jimmy McDonough in Shakey, sometime in 1975, Dylan showed up unannounced at the rented Malibu house where Neil and Crazy Horse were recording the classic Zuma LP. One thing led to another, and soon Dylan was teaching the band songs from the just-released Blood on the Tracks—though guitarist Frank Sampedro admitted to McDonough that they “never got one right all the way through.” Still, if tapes were rolling, it’d be wild to hear the results of this impromptu session, in all its ragged glory.

The Rolling Thunder Revue's Last Stand

The dogged Dylan taper community has ensured that nearly all of Dylan’s live appearances since 1974 have been recorded for posterity. But one show has managed to remain completely unheard: the final performance of the Rolling Thunder Revue, Dylan’s music-festival-as-traveling-circus, which took place in Salt Lake City in May 1976. This missing tape is especially frustrating for one reason: The setlist inclusion of “Lily, Rosemary & the Jack of Hearts,” a Blood on the Tracks epic that Dylan had never played before and has never played since. "The song has something like 18 verses," photographer and archivist Joel Bernstein, who was at the show, told the defunct '80s Dylan fanzine the Telegraph. "Bob wrote the first lines down on the back of his hand, on his cuff, on his sleeve… They did a very good version of it, really good." Though it’s never emerged, there are those in the Dylan community who swear that a Salt Lake City soundboard recording is out there somewhere. Maybe Sony is saving it for The Bootleg Series, vol. 37.

The Mid-Seventies Divorce Songs

“They were all very, very, very tough, dark, dark, dark songs,” Dylan sideman Steven Soles told biographer Howard Sounes in Down the Highway: The Life of Bob Dylan, referring to a series of tunes written in 1976 and 1977 after the songwriter’s acrimonious divorce from Sara, his wife of more than a decade. “Our mouths were just wide open. We couldn’t even believe what we’d heard.” Perhaps they were too dark, too personal; according to Soles, none of the songs he heard Dylan play from this period ever made it onto a record. They were only played privately for a few friends and associates. “I cut that whole experience right off,” Dylan claimed a few years later in an interview with the LA Times’ Robert Hilburn. “I had some songs... They dealt with that period as I was going through it. For relief I wrote the tunes. I thought they were great… But I had no interest in recording them.” So it’s possible that this particular era of Dylan’s songwriting life left no listenable trace.

The Lost Eighties Sessions with Al Kooper, T-Bone Burnett, and Cesar Rosas

The mid-1980s were a low point for Dylan as a record-maker, with two half-hearted LPs, Down in the Groove and Knocked Out Loaded, suffering from the most scathing (or just plain indifferent) reviews of Dylan’s career. But Rolling Stone's Mikal Gilmore was present for studio sessions showing that Dylan could still find inspiration if the circumstances were right. With a motley crew of session musicians including Al Kooper, T-Bone Burnett, Los Lobos guitarist Cesar Rosas, R&B saxophonist Steve Douglas, and bassist James Jamerson Jr., Gilmore writes that Dylan cut well over 20 songs ranging “gritty R&B, Chicago-steeped blues, rambunctious gospel, and raw-toned hillbilly forms.” And not a single one has seen the light of day, on bootleg or otherwise.

Jim Dickinson's Time Out of Mind

Michael Chaiken, the aforementioned curator of Dylan’s vault, mentioned another fascinating find in that Rolling Stone interview a few months back: a “completely different version” of 1997’s Time Out of Mind, produced by the late, great Memphis musician Jim Dickinson, best known for his work with the Rolling Stones and Alex Chilton. Daniel Lanois produced the finished Time Out of Mind, and its murky textures and wasted vibe earned Dylan an Album of the Year trophy at the Grammys. But the idea of the idiosyncratic Dickinson manning the boards for this moody set of comeback songs is an intriguing prospect to say the least.