If you were one of the lucky people to secure an early-bird ticket to this year's Eaux Claires Music & Arts Festival, you probably received a very special treat from the organisers in the form of a limited edition cassette featuring demos and B-sides from artists playing this year's festival.

Among the tracks was a previously unreleased Bon Iver song called 'Haven, Mass' that someone eventually shared via YouTube (you can stream it below). Justin Vernon has since tweeted that the track is six or seven years old - a lost treasure, so to speak.