According to Tom Clancy, in "The Hunt for Red October"  there is a
transmission of coded groups to very deep operating "Boomers" at a data
rate that appears to be in the region of a bit per second (one minute
per message). The groups would encode messages such as "Come to the
surface immediately for a satellite transmission", or "Fire !". If it is
not all a fiction invented by an author with a large collection of
friends working at interesting places. Speculatively of course, this
would be at a frequency in the 60 - 170Hz region - which makes even

amateur 9kHz experiments seem tame.

No secret. there have been several articles about the Us VLF sites, with
many miles of antenna wire. IIRC the frequency was about 77 c/s.

Mike
--
M.J.Powell