The hydrophone’s origin is attributed to the Canadian inventor Frank Massa in 1929, known as the Fessenden oscillator. This was a predecessor to the hydrophone, that could ordinarily be used to capture aquatic sound as a unique type of recording (listening) device. However, a part of hydrophone history is that it could also be used on underground projects such as in field research. This was implemented and re-designed by the French in World War II for detecting German sappers tunnelling underground.Since the early days of World War I, and with continuing usage throughout the years including into World War II, hydrophones were commonly used with a transducer for the detection of submarines, mine fields and navigation. Originating with the iconic ‘ping’ (pulse of sound), they have developed a long way in terms of performance but still retain the basic principles. It was Ernest Rutherford – a New Zealand born, British nuclear physicist – who pioneered the research into hydrophones by using piezoelectric devices, and this was his only patent for a hydrophone device. The acoustic impedance of these piezoelectric materials facilitated its use as underwater transducers. The piezoelectric hydrophone was implemented late on in WW1 by merchant shipping naval escorts that had the ability to detect german u-boats, greatly impacting the effectiveness of submarine attacks shortly after the sinking of the R.M.S. Lucitania in the Atlantic, on 7th May, 1915.