Noun | hy-drone-phone | \hi-drōn-fōn\ : An Instrument for listening to sound, mounted on an unmanned underwater vehicle. Yes, I made that word up—you heard it here first! The acronyms “UUV” (Unmanned Underwater Vehicle) and “AUV” (Autonomous Underwater Vehicle) have been buzzing around the underwater community recently, along with the term “underwater drone,” which gained popularity in the media due to an international incident in December 2016 regarding an ocean glider (a subclass of AUV). Unmanned underwater vehicles are quickly becoming ubiquitous in the world of oceanographic sensing, military operations, and oil and gas exploration. Because sound travels far and fast underwater, it is an enabling mechanism for navigation and communication for these vehicles. “Hydronephones,” specifically ocean gliders, are being used by our underwater acoustics community as receivers for long-range acoustics and marine mammal sensing. Key advantages of these vehicles are that they can be deployed for long durations of time in harsh environments and that they are much more cost-effective than traditional ship-based observational methods. An overview of this exciting technology will be presented along with details of some recent and ongoing “hydronephone” projects in underwater acoustics.