A while back, Foo Fighters' Dave Grohl dropped the news that "probably the biggest pop star in the world" contributed backing vocals on the band’s upcoming album, Concrete and Gold. Ever since then, Grohl has been asked about that mystery star over and over.

In a new interview with Rolling Stone, Grohl said he wouldn’t reveal the guest, but he did elaborate.

To be clear, Grohl wouldn't use the phrase “pop star” again in regard to the mystery guest. "This person is, I think, is more than that," he said.

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As for how this person got involved, Grohl explained it was a last-minute sort of thing.

"It was sort of towards the end of the album and this person came up and said, 'Man, I'd really love to sing on your record,'" Grohl said. "I looked at (Concrete and Gold producer) Greg (Kurstin) and said, 'OK, come in tomorrow and we'll figure something out.'

Grohl also said that this person has "been around a long time," and he would "be amazed if anyone can really figure it out."

"I think I've met this person maybe a few times, but I'd never spent time with this person, and we had a blast together," he added. "And (this person) very talented -- more so than I ever knew."

Concrete and Gold will arrive on Sept. 15.

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