With production of the next Foo Fighters album up in the air, frontman Dave Grohl is exploring his metal mentality with Probot, a collaborative side project involving heavy metal luminaries King Diamond, Tom Araya of Slayer, Motörhead's Lemmy Kilmister and Thomas Gabriel Warrior of Celtic Frost, among others.

The project started about a year ago, when Grohl and Adam Kasper (engineer, mixer and producer of the Foo Fighters' 1999 album, There Is Nothing Left to Lose) recorded seven instrumental metal tracks in Grohl's basement studio as something of a response to the LP's first single, "Learn to Fly," which the former Nirvana drummer described as "the most middle-of-the-road piece of sh-- I've ever written in my life."

"[I thought,] 'It's so boring,' and 'I can't believe it's the first single,' and 'It doesn't represent the rest of the record,' " Grohl recalled. "So I was just kinda itching to record something that I was really excited about. I love the Foo Fighters record, but there's a part of me that's always been kind of this metalhead kid."

Inspired by Santana's Supernatural, Grohl set out to make the "death metal" version of the multi-artist project by first coming up with a wish list of artists to contribute to his instrumental tracks, then sending out tapes to all who agreed to participate.

Grohl, who claimed metal was a natural progression for him after growing up with bands such as Bad Brains, D.R.I. and Corrosion of Conformity, admitted he was a bit surprised at the response from his heavy metal heroes.

"I consider myself such a stupid, middle-of-the-road, alternative-rock idiot, I thought most of these people would be like, 'No, he's a dork' or 'No, he's a jerk,' " Grohl explained. "But they all agreed to do it."

So Grohl and Kasper, along with friend and Chavez frontman Matt Sweeney, re-entered the home studio and laid down five more instrumental tracks to round out the project.

Other artists on the receiving end of the Probot tapes are Cronos from Venom, Eric Wagner (Trouble), Lee Dorian, Snake (Voivod) and Wino (Obsessed/Spirit Caravan).

Whether Probot's "death metal Supernatural" will ever surface remains to be seen. At the very least, Grohl said his metal foray is likely to impact the next Foo Fighters album by making it "a little harder, sludgier." If nothing else, Grohl is having a blast creating it.

"It's kind of a dream come true," Grohl said. "It's like going to one of those stupid baseball camps where some executive businessman gets to play with the pros."

(Iann Robinson also contributed to this report.)