As previously reported, Bon Iver’s Justin Vernon and the National’s Aaron Dessner are linking up to throw the second annual Eaux Claires Music & Arts Festival in Eau Claire, Wisconsin this summer. Although it hasn’t been officially announced yet, the duo sent a cassette featuring b-sides and demos from artists on the bill to fans who bought early bird tickets to the fest. One of those b-sides features a previously unreleased Bon Iver track, which immediately found its way online. According to Justin Vernon, “Haven, Mass” is “very old,” probably from around “2009 or 2010,” before the band released their self-titled album in 2011. Listen below.