> At extremely low frequencies (that's the technical term...) radio waves can reach submarines that are submerged below a few m depths. > VLF [...] https://en.w

> At extremely low frequencies (that's the technical term...) radio waves can reach submarines that are submerged below a few m depths. > VLF [...] https://en.wikipedia.org/wiki/Jim_Creek_Naval_Radio_Station https://en.wikipedia.org/wiki/Jim_Creek_Naval_Radio_Station Even the signal from https://en.wikipedia.org/wiki/VLF_transmitter_DHO38 https://en.wikipedia.org/wiki/VLF_transmitter_DHO38 can "officially" reach around 30m depth in seawater worldwide.