Here's a plot device I've used a couple times in espionage games:
Project ELF. It was started in the late 1960s as a system for mantaining radio communication with the US submarine fleet by using Extremely Low Frequency radio waves. The system was based in northern Wisconsin because the geology of the region was condusive to the ELF waves. The system went operational in the mid-'80s, but was subject to much contorversy. It was shut down in 2004.