‘Sound underwater and underground’ focuses on the areas of sound that we can’t hear. It was not until World War II that the richness of the underwater soundscape became clear. Listening with hydrophones revealed a cacophony of sounds extending well above and below the human frequency range. From the 1910s, primitive hydrophones were gradually replaced by echo sounding, from which modern sonar systems developed. Sound waves that travel through the earth can be detected with geophones, which are highly directional and are usually deployed to respond to sounds from directly below. Modern microelectromechanical systems are relatively insensitive and are used mainly for the monitoring of actively seismic regions.