In the shadowy corners of the shortwave radio spectrum, you can often find mysterious mechanical voices counting off endless strings of numbers — in English, Czech, Russian and German … even Morse code. But who's listening?

The voices are coming from what are known as "numbers stations," and they've long been thought to be part of international espionage operations. In fact, the Russian spies recently captured here in the U.S. may have been getting orders from Moscow via a shortwave numbers station.