World's slowest concert won't finish for another 614 years

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 piece of music moves so slowly in fact, that some musical notes last for years before they change.

The unusual concert began in September 2001, but it started with silence.

The first chord wasn't heard until nearly a year and a half later, in February 2003.

Earlier this week, the music finally reached its 17th chord change.

More than 150 people from all over the world came to the venue to watch as organisers carefully added a new pipe to the organ, taking the sound from seven notes to eight.

The musical instrument has been specially designed to keep playing without stopping.

An electric blower sends air through the pipes, while sandbags hold down the keys.

When it's time for a chord change, people have to add or remove pipes by hand.

The long performance is expected to continue for 639 years.

The length was chosen because the town where the concert is taking place had a famous organ dating back to 1361- exactly 639 years before work on the concert began.

The next musical change will take place in October 2027, when one of the notes will be removed from the chord.

The concert is finally due to finish on 4 September 2640.