It can be fascinating to hear what goes on below the surface of the sea or even rivers and ponds. This is exactly what a Hydrophone (Underwater Microphone) can deliver to your ears – in that case through my good old Zoom H2 audio recorder. The fact that the H2 has a Plug in Power source on board makes it suitable for the excellent Aquarian Audio Hydrophone.
And yet a Hydrophone is not restricted to natural masses of water – some people do experiments in swimming pools and there is a video where a guy records inside a blender (some centimeters away from the blades of course).