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- Submarines evolved from primitive Civil War vessels to advanced WWI diesel-electrics, enabling stealth attacks.
- Early radio communication with submerged subs failed due to seawater blocking HF, forcing surfacing for messages.
- In 1917, Willoughby discovered VLF (3-30 kHz) antennas worked underwater, leading to Navy adoption for deep reception (up to 100 feet).
- VLF systems like NSS (1918) and Cutler (1961) used massive antennas and high power (e.g., 1.8 MW) for one-way Morse code commands.
- Nuclear submarines (1954 onward) required deeper communication, driving ELF (72-80 Hz) research for 'bell ringing' alerts.
- ELF faced technical limits (low data rate, huge antennas) and political opposition, leading to the scaled-down Project ELF (Clam Lake & Republic, 1989).
- ELF was shut down in 2004 due to advances in VLF, high costs, and public controversy, though China, India, and Russia operate similar systems.