Robert Plant has revealed that “Polly Come Home,” one of the covers he recorded alongside Alison Krauss for 2007’s Raising Sand, ranked among the most challenging songs of his career.

“It’s just the most difficult piece of music to sing at the tempo that we sang it at,” Plant explained on the latest episode of his Digging Deep podcast. “It’s one of the toughest calls I’ve had, apart from my audition in the Yardbirds.”

While Gene Clark’s original version of the song -- released in 1969 on the Dillard & Clark LP Through the Morning, Through the Night -- was a mid-tempo country tune, Plant and Krauss opted to slow the pace down to an intoxicating sway.

Listen to "Polly Came Home" by Robert Plant and Alison Krauss

“It’s such a remarkable feel and it’s such a beautiful song,” Plant remarked. “The song itself is just, it’s so poignant. And it’s so slow. 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.”

“So this is the guy that sang ‘Immigrant Song,’ he’s just gonna go and sing this song here where I need an iron lung to give me a little more air to get the song out,” Plant continued. “It’s just such a great song, but the tempo… it was so languid, it was magnificent. But it was a hell of a challenge.”

Of course, Plant reaped plenty of rewards for challenging himself. Raising Sand sold more than a million copies in the U.S., while critics celebrated the classic rocker and Krauss for their amazing musical chemistry. The LP earned the duo five Grammys, including Record and Album of the Year. The musicians are reportedly prepping a follow-up, though details have been scarce.

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