Strange pipe

Pipes are always my favourite objects in underground mines. You can manipulate it whatever you want, it will do different sounds everytime. That's why since a year now, I'm especially researching mining pipes. This one is fun, like many other. It's a vertical pipe, buried in the ground. At the bottom, there's water. I put microphones inside the pipe (about one meter) and I'm droping very small gravels. Pipes are all my life ;-)