The title of this year's release, which ties in with a documentary series called Sonic Highways, hasn't yet been confirmed. The mainman previously described it as their most ambitious to date.

He later revealed that each of the tracks was recorded in a different studio, with each episode of the documentary tying to one song. Now he's added that it's probably their longest work to date.

But even before work had been completed Grohl had come up with another concept. He says: "I already know what we're doing for the next Foo Fighters project - and that's even f***ing crazier.