As you probably know by now, Dave Grohl has been working on a documentary about the Sound City Studios in Van Nuys, CA (where many classic albums, like Nirvana’s Nevermind, were recorded). And for the past few weeks, it would appear that the Foo Fighters frontman/former Nirvana drummer has been doing some unique viral marketing.

As seen in a series of photos posted on the documentary’s Facebook page, Grohl (or someone else associated with the film) has been leaving copies of the mix tape seen above. It would appear that copies of the mix tape were left in numerous places during SXSW and most recently in Chile, Argentina, and Brazil (at least that we know of). The mix tape itself features songs from Metallica, Dio, Kyuss, Mastodon, Nine Inch Nails, Cheap Trick, Ratt and numerous others.

Of course, it’s no coincidence that all of those bands are on the same mix tape. At least one member of each group on the mix tape was seen or mentioned in the documentary’s trailer (although we somehow missed Mastodon and Neil Young). Plus, we know that Grohl recorded music with many of these musicians, including Corey Taylor and members of Ratt, for the film. Could the mix tape’s track listing be hinting at other collaborations Grohl recorded, or just a tease of who else was interviewed? All we can do at this point is speculate, but it none the less has us excited to see the movie.