Quite a few VLF/ELFers attempt to copy the Russian ZEVS transmissions to deeply submerged nuclear subs at around 82Hz, yes, you read right, Hertz. At these frequencies even a few watts ERP penetrate deeply in the sea. The Americans used a similar system called Project Sanguine at around 76Hz , but this has now gone QRT. I think they use green lasers now?
The ZEVS data rate is low and usually there are a couple of transmissions a day. Usually Spectrum Laboratory software is needed for long term stability. Several people have been successful, although I have still to try.
I hope the transmissions are just tests these days!