Despite having one of the most distinct voices in rock 'n' roll, Robert Plant still finds singing some songs challenging, and has named his absolute "most difficult" song to sing.

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No, it's not a Led Zeppelin song — though tackling one of those would be quite a task for almost anyone. It's his cover of Dillard & Clark's "Polly," which he and Alison Krauss re-recorded as "Polly Come Home" for their collaborative 2007 album Raising Sand.

Plant explained the song's complexity during a recent episode of his Digging Deep podcast.

“It’s just the most difficult piece of music to sing at the tempo that we sang it at,” he said [via Ultimate Classic Rock]. "It’s one of the toughest calls I’ve had, apart from my audition in the Yardbirds.”

“The song itself is just, it’s so poignant. And it’s so slow," the singer continued. "So the very opening line of the song, in my chest, my lungs, my vocal cords, in my sense of timing... It was, ‘How am I gonna get these words right to the end of that bar without collapsing?’ It was just such a beautiful lilt.”

Referring to himself as "the guy that sang 'Immigrant Song,'" he joked that he needed an iron lung in order to finish "Polly Come Home."

Listen to the track below.

Speaking of Led Zeppelin, the very first authorized documentary about the band — Becoming Led Zeppelin — has officially been completed, according to Variety. The film, which does not yet have a release date, will feature new interviews with Plant, Jimmy Page, John Paul Jones and archived footage of the late John Bonham.

Becoming Led Zeppelin is a film that no one thought could be made,” said director Bernard MacMahon. “The band’s meteoric rise to stardom was swift and virtually undocumented. Through an intense search across the globe and years of restoration of the visual and audio archive found, this story is finally able to be told.”

Robert Plant + Alison Krauss, "Polly Come Home"

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