Excellent Youtube & Description - Submarine Communications

In this March 2018 Youtube Professor Simon Holland provides an excellent presentation, with graphics, on submarines comms.
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Youtube above - at about 3min 35secs Extremely Low Frequencies (ELF) is raised. with the ability to reach subs at their safe operating depths, maybe 500 meters down. ELF transmitters need to be very long (rather than very high) and on special geologic environments. The USN has ELF antennas
-  Clam Lake ELF Transmitter Wisconsin to Michigan (US mainland)
-  Hawaii
-  6min 15sec Daventry UK (for USN? UK Royal Navy and other NATO? subs)
-  INS Kattabomman in southern India (Indian Navy and USN?)
-  Naval Communication Station Harold E. Holt in north Western Australia for USN, UK RN? and
   RAN
-  6m 35s a "secret" one in Antarctica, breaking the international agreement that the Antarctic not
   be used for military purposes (probable cover use is "Research"?)
-  6m 55s ELF transmitters dangerous to health?
-  8m 5s dangerous to passing Qantas passenger jets?
-  8m 30s irritating whales?
-  9m 10s back to submarines
-  9m 25s underwater communications docking stations
-  9m 40m satellite based system, known as Submarine Satellite Information Exchange Subsystem
   (SSIXS)

Diagram (above) courtesy Daniel Olson's, October 11, 2015 excellent description on the Clam Lake WI Extremely Low Frequency (ELF) Transmitter  [to submarines] website. Part of Daniel's description is:

"[During the early years of the Cold War, late 1940s on, the] U.S. started to look at weapon systems that would be able to survive any type of attack without the loss of function. The U.S. Navy discovered that weapons systems that operated mainly in the ocean waters would be the most resilient. [Following Polaris, used from 1961.]  This concept was the basis on why the U.S. Navy enacted the TRIDENT weapons system used on the U.S. nuclear submarines in 1979 (Klessig 4). 

The U.S. Navy looked at extremely low frequency (ELF) radio transmissions as a way of communicating to these weapons systems from within the country’s boundaries. The use of ELF transmissions would allow for the U.S. Navy to keep their nuclear submarines submerged and still be able to receive a message without being susceptible to attack from enemy ships or aircraft (Kruger 1).

Researchers had started to come up with ideas that certain types of radio waves could penetrate ocean depths in the 1950’s. The U.S. Navy then started its own research that dealt with trying to produce an ELF radio wave that could penetrate into the depths of the oceans. The Navy produced three projects using the concept of ELF transmissions: Sanguine, SEAFARER, and ELF (“Extremely Low Frequency” 2001)..."

See much more on ELF principles and history (Projects Sanguine and SEAFARER) on Daniel Olson's website.