When you glance at a musical composition looking like this...
...the first expectation would be that it probably sounds as if a million toddlers were banging on about as many pianos, but weirdly enough, it doesn't sound bad. It does take a certain person to listen to it, so if you're up for a challenge, continue.
This genre of unplayable music is called Black MIDI. The term comes from the MIDI (Musical Instrument Digital Interface) technology standardized in the recording industry in the 1980s which essentially allows devices and digital musical instruments to talk to one another. The other half of the phrase is from the visual sheet music of the compositions which appear to be blacked out just because of all the notes.
If you haven't Google searched it already, I'll save you the trouble of linking some of my more favorite ones throughout. Here's perhaps my favorite song from the genre, Fractal Images. Although technically this piece has 1 million notes, what I like most about this one is that it's a lot more straightforward visually and melodically. If you can't handle flashing color, I would advise just to listen.
Trying best to describe the genre, which isn't easy, it sounds like an orchestra of piano parts layered on one another and only played by a single piano, sped up at least twenty times any other reasonable piano piece could be played, with synth-like sounding combination of notes for the purpose of having a beat or other electronic sounds you wouldn't think could be mimicked by a piano at all.
A big part of the genre is the visual aspect of the notes being played. The sheet music would just be black, so a visual program such as Synthesia is used a good amount of time. Some compositions are so hardcore, that the screen for that gets blacked out as well.
It definitely took me a few, I would say, "transitional" pieces to even be able to listen to the genre. It's semi overwhelming visually and auditorily at first. I have found that the lower the note count (I'm saying "low" when I mean around 6 figures), the less experimental it is. I actually stumbled upon one of the many remixes of Bad Apple and was not ready to hear it and eagerly clicked away.
What made me click back to this space several months later is the fact that the largest on of these songs has over a trillion notes and is something like 4.2 terabytes in its raw form. It blows my mind to think that human ingenuity has created programs and hardware to handle that sort of massive file, as well as have someone actually create a file.
If you're up for something a little bit more aggressive, I also liked The Nuker 2. It's longer, but these songs aren't like pop songs where each chorus is pretty similar to one another, each minute is different, each minute is a constant musical journey. With 142.2 million notes, this is one of those songs that black out at 5:20:
Enjoy.