Hidden among the squelch and whine of the little used shortwave radio band, mysterious stations broadcast unbreakable code.

Yosemite Sam threatens to blow the listener to smithereens before switching to a different frequency. An upbeat woman delivers nonsensical strings of numbers in Mandarin. A repeating broadcast of a nursery rhyme breaks only for a child to read numbers in German.

These are the numbers stations—a radio station on shortwave that broadcasts some sort of repetitive noise followed by strings of numbers. Amateur tech geeks first identified the stations after World War II. No one is sure what their purpose is.

That hasn’t stopped anyone from speculating. The most popular theory is that the broadcasts are used to transmit coded messages to spies and the military. Shortwave is easy to broadcast globally, hard to trace and free of commercial traffic.

Spies or military personnel tune into the frequency at an appointed time and use a one-time pad to decrypt the message. The spy then destroys the pad and goes about their mission. Anyone else listening hears a random string of numbers with no context.

Listening to numbers stations was once the hobby of a small margin of the population. Only those with shortwave radios and patience to tune them reaped the benefits of the strange broadcasts.

Now—thanks to the Internet—anyone can listen.

The University of Twente in the Netherlands maintains web-based shortwave radio anyone can access here. The interface is simple. Just input the frequency you want in the box below the graphic.

To get an idea of what frequencies to check out, head over to Priyom.org—an international group researching intelligence and military communications via shortwave radio. The site maintains a schedule of active shortwave stations and catalogs interesting activity.

If you find an interesting broadcast, post it to our Facebook page. To get you started, here are two of War is Boring’s favorite numbers stations.