"DO NOT ANSWER": 5 Mysterious Radio Signals and Numbers Stations

In 2012, a number of radio listeners in America heard a mysterious transmission that would prove, as investigations unfolded over the following years, to have wide-reaching geopolitical implications.

The frequency usually broadcasts nothing but static silence. However, on this occasion, and at intervals thereafter, a masked voice was heard. The voice was that of a woman, robotically distorted, reading seemingly random numbers in Spanish.

This was followed by a number of digital tones…

… which then faded away. The recorded message came in waves, repeating strings of digits, for about 20 minutes before they ceased and were replaced by the crackle of yet more static.

The most popular explanation for the signal is that a government agency was distributing coded messages to spies. Spies with the key to decrypt the messages would receive their orders by listening in and deciphering the patterns of numbers.

Investigations found that the signal was being broadcast by a station codenamed HM01, which is indeed owned by a Cuban intelligence agency. However, it remained unclear what the messages meant or for whom they were intended...