I’m reminded of https://en.wikipedia.org/wiki/Communication_with_submarines#Extremely_low_frequency https://en.wikipedia.org/wiki/Communication_with_submarines#

I’m reminded of https://en.wikipedia.org/wiki/Communication_with_submarines#Extremely_low_frequency https://en.wikipedia.org/wiki/Communication_with_submarines#... where there’s just enough of a channel through the ocean to send a few symbols that mean something prearranged like “make contact ASAP.”