Hydraulophones
The first three Hornbostel Sachs numbers describe instruments that make sound from matter in its solid state. The fourth HS number describes instruments that make sound from matter in its gaseous state (air). The fifth HS number describes instruments that make sound from electricity.
Recently a number of instruments have been invented, designed, and made, that make sound from matter in its liquid state. This new class of instruments is called hydraulophones. Hydraulophones use incompressible fluid such as water as the initial sound-producing medium and may also use the hydraulic fluid as a user-interface.
Read more about this topic: List Of Musical Instruments By Hornbostel-Sachs Number, Instruments That Do Not Fit Into Any of The Above Five Categories