Extreme Low Frequency (ELF) radio signals are considered useful for one-way communication to submarines because they penetrate seawater to depths of several hundred feet. ELF is the only available system which, according to the US Navy, will allow continuous contact with submarines while they are at patrol depth and cruising speed. The history of ELF started with Sanguine which was to be a hardened system comprising over a hundred buried transmitters and 6,000 miles of underground cable covering 6,500 square miles in Michigan and Wisconsin. When that wasn't accepted the Navy proposed SEAFARER (Surface ELF Antenna For Addressing REmotely-deployed Receivers) with three above-ground transmitters and 2,400 miles of buried cable occupying 4,700 square miles. Then it was scaled down to Austere ELF with two surface transmitters and 158 miles of cable -- 130 miles of it buried along utility and roadway right-of-ways. The final incarnation was Project ELF, still with two transmitters and only 84 miles of cable hung on utility poles. In 2004 the US Navy ended Project ELF, citing improvements in communication technology that rendered ELF obsolete as well as the changing requirements of the modern Navy.