When Dave Grohl appeared on stage at Nashville's Bluebird Cafe earlier this month, it was a huge surprise for the audience. But Grohl offered no explanation for his unannounced performance, or for the fact that video cameras were filming the entire time.

It turns out, Grohl was there as part of an upcoming HBO series. Grohl's band, the Foo Fighters, have announced that they have been working on a new album in eight cities in the US, including Nashville, with HBO filing them for the upcoming project.

Grohl is directing the series, which features the band tapping "into the musical heritage and cultural fabric" of Chicago, Austin, Nashville, Los Angeles, Seattle, New Orleans, Washington, DC and New York. They recorded one song in each city, working at local legendary studios for their eighth studio album.

Each city will be featured in its own episode of the upcoming series, which is slated to air this fall. The new album is also set for release in the fall.