Led Zeppelin frontman Robert Plant has revealed the “most difficult” song he has recorded.

The song is “Polly Come Home,” one of the covers he recorded alongside Alison Krauss for 2007’s Raising Sand. Gene Clark‘s original version of the song was released in 1969 on the Dillard & Clark LP Through the Morning, Through the Night.

“It’s just the most difficult piece of music to sing at the tempo that we sang it at,” Plant revealed on the latest episode of his “Digging Deep” podcast, according to 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.”

“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.”

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