Portal 2 Turret Song A Capella

An A Capella group performed the turret song from Portal 2 - it's one of the most beautiful pieces of music I've heard, bar none. No spoilers, either.
http://www.youtube.com/watch?v=gFCKhUgI ... dded#at=77
http://www.youtube.com/watch?v=gFCKhUgI ... dded#at=77