What is this? For more than 30 years, shortwave radio bands around the world have been haunted by “numbers stations” on which anonymous voices recite strings of numbers and letters. These stations transmit in various languages, following strict schedules, but they never identify themselves or give any hint as to their purpose. It seems likely that they’re run by government agencies, sending messages to spies in the field using a prearranged code. But why does the station known as “The Buzzer” send out buzzing sounds on 4625 kHz continuously, throughout the year? And why on earth does the station recorded above, known as the “Swedish Rhapsody,” transmit the sound of a music box and a little girl’s voice?

Numbers stations are beyond the reach of the Freedom of Information Act because (presumably) secrecy is essential to their missions, and in fact in the United Kingdom it’s illegal even to listen to them. So we’re unlikely to learn the full story anytime soon. But in 1997 the Irdial-Discs record label assembled a 5-CD set of recordings and has made it freely available to those who want to study them.