1) Extremely low frequency - https://en.wikipedia.org/wiki/Extremely_low_frequency
2) Communication with Submarines - https://en.wikipedia.org/wiki/Communication_with_submarines
An important addendum is the subject of "Underwater Acoustic Communication" - https://en.wikipedia.org/wiki/Underwater_acoustic_communicat...
RF Science is fascinating. Now might be a good time to ask; Does anybody have any recommendations on scientific books/papers/articles on the effects of RF spectrum (all frequencies) on Human Biology and Physiology? There is a lot of nonsense/hysteria out there but i would like to know the actual experimental evidence and science.