If anyone is still digging on 17Hz signal that went around the world:
17 Hz is considered Ultra Low Frequency.
Previous known systems for sub comms Voice Frequency (VF) 300-3000 Hz, Very-Low Frequency (VLF) 3-20 kHz, Extremely Low Frequency (ELF) 30-300 Hz
Did ELF give way to ULF?
Bunch of ELF history and links below:
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ELF Project Sanguine – planned 6000 miles of antenna in Wisconsin, 100 transmitters buried 5 feet deep. Killed 1973 by Melvin Laird, Sec Def
Project Seafarer – Revived 1975 in Michigan, 2400 miles antenna with transmitter in Michigan at KI Sawyer AF Base, killed by Carter 1979
Project ELF – Greenlighted by Reagan 1981. Two sites with arrays: Clam Lake Chequamegon National Forest Wisconsin (went live 1985), KI Sawyer AF Base in UP Michigan (went live 1989) 165 miles away. Use precambrian bedrock base as part of the antenna. Decomissioned 2004
ELF overview
http://www.navy-radio.com/commsta-elf.htm
https://archive.fo/FpEkn
History and building it:
https://fas.org/nuke/guide/usa/c3i/fs_clam_lake_elf2003.pdf
https://archive.fo/jR06m
https://www.hep.wisc.edu/~prepost/ELF.pdf
https://archive.fo/elYYx
http://ss.sites.mtu.edu/mhugl/2015/10/11/clam-lake-wi-elf-transmitter/
https://archive.fo/STiBX
Decomissioning: https://www.ar15.com/forums/general/Navy_prepares_for_Wisconsin_ELF_station_shutdown_on_Thursday/5-278894/
https://archive.fo/e2nYG
Technical on ELF:
https://fas.org/man/dod-101/navy/docs/scmp/part07.htm
https://archive.fo/ZQa1R
Much more to dig on.