Numbers station

A Numbers station is a kind of shortwave radio station with unusual broadcasts: very often, these broadcasts consist of numbers that are read by computer-generated voices. Several of these stations exist, numbers are read in different languages. Some stations have operated for a period of more than ten years, using the same frequency. 
The purpose of the stations is unknown, along with who operates them. One supposition is that governments use them to communicate with secret agents or spies. As receiving the messages is relatively simple, the agent only needs a world radio receiver.