What's the longest concert you've ever been to? A couple of hours? Maybe a whole day at a music festival? Well, in Germany, one concert has already been playing for nearly 25 years - and it still has more than 600 years left to go before the final note is heard. The unusual performance is taking place inside a church in the north-east of the country, where a small organ is playing the same piece of music very slowly. This week, music fans gathered inside the venue to hear a rare chord change - a musical moment that only happens once every few years. The performance is taking place in the medieval Burchardi Church, in the town of Halberstadt, and is based on a piece of music by American composer John Cage called 'ORGAN²/ASLSP' - short for 'As SLow aS Possible.' The concert is expected to continue for 639 years and is finally due to finish on 4 September 2640.