Sigur Rós improvise a song on a stone marimba in the Surtshellir caves in western Iceland.  They made the marimba themselves from stones collected there.  From Heima.

It just so happens that we (fellow ahogados and I) were hiking through those caves not a month ago.  It all seems warm and cozy in there from this video.  Sweaters and smiles by the fireside. However, we had the opposite sensation — that it was haunted.  Pitch, pitch black, three lanterns could barely light our immediate surroundings, and the cave floor was a deadly combination of slippery ice and jagged rocks.  Full of strange noises and uneasy silences, and even cliffs overhead, it was definitely ruled by either unfriendly trolls or a clan of ice-loving ghouls.  This video for me is a testament to how a bit of beautiful music (ok, and some torches) sends the darkness running, and can transform even a ghoul lair into a cheerful, welcoming refuge.