When Universal Music Group announced last week that it was re-releasing the Rolling Stones’ iconic 1972 double album “Exile on Main Street” with 10 never-before-heard bonus tracks from the period, many fans found themselves wondering, “Where did these songs come from, and why haven’t we heard them before?”

By all outward signs, the Stones’ music has been one of the catalogs most assiduously strip-mined by both the various labels they’ve been associated with and by enterprising bootleggers who tout thousands of hours of the band’s recorded and live work dating back to their formation in 1962. One bootleg site boasts eight CDs of material from the “Their Satanic Majesties Request” sessions alone.

Don Was, the credited producer on the newly unearthed “Exile” tracks along with the late Jimmy Miller and the Glimmer Twins (the Stones’ Keith Richards and Mick Jagger), told Daily Variety that he sifted through 200 hours of tape for the new bonus material, which will be heard when the package is released May 17 in the U.K. and May 18 Stateside.

“I’ve been to the tape warehouse,” Was said. “If you can picture the last scene of ‘Raiders of the Lost Ark,’ with this enormous room filled from floor to ceiling with (all these antiquities), that’s what they have.

“There’s stuff that nobody’s heard,” he added. “The things that have slipped out on bootleg is just a fraction of what’s available. They could do this with every album they’ve ever made if they wanted to.”

Was said nobody was more surprised than the Stones “that they had this extra stuff that was as close to the finish line as it turned out to be.”

The original 18-track double album — made during a peak creative period for the band and the culmination of a string of masterpieces that began with “Beggars Banquet” and continued with “Let It Bleed” and “Sticky Fingers” — was recorded in various stages at multiple locations, including Olympic Studios in London, Keith Richard’s Nellcote mansion in the south of France and Sunset Sound Recorders in Los Angeles, where the bulk of the mixing and overdubs were done. The “Exile” of the title pertains to the Stones having moved out of England due to the country’s onerous tax laws and lingering legal matters traced to drug busts in their home country.

It is considered a rock ‘n’ roll landmark, with blues, country, R&B and gospel thrown into the mix. Blues and slide guitar specialist Mick Taylor, who replaced Brian Jones in the band after he died in 1969, is considered a key part of the album’s timeless appeal.

“There’s a unity to ‘Exile’ that makes a whole lot of sense — this vision of American roots music reinterpreted by the Stones’ eyes and ears,” Was said. “And it holds because it’s stayed current all these years.”

Jagger was reportedly critical of the album at the time of its release and was quoted in 2003 as saying the LP “has some of the worst mixes I’ve ever heard.”

Was said that he and the Stones (Jagger, Richards and, to a lesser extent, drummer Charlie Watts) decided to maintain the integrity of the original release. “The sound of ‘Exile’ is not only etched in stone, but it’s become a part of the sonic vocabulary of recordmakers who followed,” he explained. “You have to embrace what it is.

“The guiding light came in a fax Keith sent to me early on. He said: ‘You don’t have to make it sound like ‘Exile.’ It already is ‘Exile.’ That’s really what we tried to stick to throughout.”

Was described the bonus tracks, which include alternate versions of “Soul Survivor” and “Loving Cup,” as either “extremely finished songs” or “finished but with a rawness.” One track, “Following the River,” consisted of just an instrumental on which Jagger subsequently wrote lyrics and supplied the vocals. But for the most part, Jagger and Richards’ overdubs are “pretty minimal” and limited to a couple of songs.

Accompanying the release is a 61-minute doc about, as director Stephen Kijak describes it, “why the Stones went into exile in France and about how they made this extraordinary album.”

Kijak culled his footage mainly from about 20 hours of outtakes from Robert Frank’s infamous documentary of the Stones’ 1972 tour, “Cocksucker Blues,” as well as images from the lavish, limited release photo book by Dominique Tarlet called “Exile,” which chronicled the recording sessions at Nellcote.

“We let Robert Frank’s aesthetic, and the album art itself (shot by Frank), lead it,” explained Kijak. “So what you’ve got is a vibey mood piece that dips you right into the early ’70s and doesn’t let you out.”

The 18 tracks from “Exile” provide the doc’s soundtrack, along with studio chatter from the “Exile” sessions as well as the more recent reissuing and remastering sessions.

Kijak also says the film doesn’t shy away from Richards’ reported struggles with heroin at the time of the recordings. “You can’t separate the drugs from the story,” he explained. “The first thing that Anita Pallenberg (Richards’ girlfriend at the time) said to us was “we were into drugs for breakfast, lunch and dinner.”

“End to end, the film is probably the most concentrated collection of rare, never-before-seen imagery on the Stones you’ve seen in a good long while,” said Kijak.