This is the most interesting musical instrument I've seen. There are a ton of videos going into a lot more detail on his channel.
it's worth keeping up with the newer videos. Many parts have been designed and built by the community and he just got it to handle something like 20K marbles without dropping any.
It's also an excellent example of second-system syndrome.
He's currently up to 10,000 marbles. I shared this video because I think it's currently the best introductory video since the more indepth videos are great but don't really show off the music as much.