The hydraulis is an ancient musical instrument, often considered the predecessor of the modern organ. It uses water pressure to create sound, with air being forced through pipes by a system of bellows. This unique mechanism allows for a range of tones and dynamics, making it popular in ancient Greek and Roman entertainment. Developed around the 3rd century BCE, the hydraulis was typically played in public spaces, such as theaters and arenas. Its design influenced later keyboard instruments, and it is often associated with the work of the Greek engineer Ctesibius, who is credited with its invention.