One of the most fascinating aspects of Cold War spy craft is the most innocuous-sounding: numbers stations. They broadcast seemingly random strings of numbers or letters over shortwave frequencies. The broadcasts are received by agents embedded in other countries. Because the signals are one-way, spies are able to hear and decode the messages without fear of radio-tracking.

Originating during World War I, numbers stations proliferated during the Cold War, that period of tension between eastern and western powers from 1947 through 1991.

In the novel Distant Early Warning, Wil and Glenn Andersen believe these broadcasts are aimed at a spy operating in their neighborhood.

This is what the boys might have heard on their shortwave receiver. It is a recording of a woman reading a string of numbers in German. The numbers were often read in groups of three, four, or five. The file comes from The Conant Project on SoundCloud. The Conant Project consists of dozens of recordings of numbers stations from around the world, available for listening or download. (For a look at contemporary use of the stations, see the Numbers Stations Research and Information Center.)