While no "Bond" here, there sure are some spies on HF! If you have tuned around the HF bands, mainly outside the Ham Radio Bands long enough, there is a chance you may have encountered a few odd things. Sometimes these are just distant shortwave stations, generally in something other than English. Other times, odd digital signals. But if you are lucky enough you may have encountered a "Numbers Station". For the most current list see: priyom.org
Basically, if you stumbled upon one of these stations (or looked them up above), you likely heard a set of the number being read by a man or woman, often synthesized voice. An example would be "45360 77334..." and often repeated a few times. The sets of numbers can vary as well as the language. Spanish seems to be commonly the strongest at least in my area last I checked. Not a huge surprise given the Cuban Intelligence (DGI) is very active on the air and is relatively close. Those on the East coast of the US, EU or Africa should have no issues either hearing these signals with a long wire antenna.
There are many Slavic languages, German and so on also heard. Worth noting that CW (Morse Code) is also used at times and there are even cases when digital modes, including RTTY have been used.
These sets of numbers are at times followed or preceded by music which perhaps makes them even odder. And yes, as you may have already figured this out, it likely has to do with intelligence agencies at work...spies! There are theories about some of these being telemetry data, etc. You draw your own conclusion.
As a side note, those who are video game fans likely encountered the opening sequence of "Call of Duty: Black Ops". This is exactly what we are talking about here. If you have not seen it, check out this clip on YouTube: https://www.youtube.com/watch?v=98ESCg4rLmE
As far as decoding these...well...good luck with that! They are technically impossible due to how it is done and perhaps this is why they are still used. Often there is a one time use key/decoder pad therefore even if you by some miracle crack one, the next will be completely different. Some may use the shift method, substitution method, some might use other systems and/or a combination of many. Making it very hard if not impossible to decypher.
Some of the most notable historical stations were during the cold war, so apparently the Call of Duty: Black Ops. above is not that far off! Another interesting fact is, many have nicknames usually due to something played alongside, geographical location or unique accompaniment, some of which I will go over.
The best known is perhaps the one which even has a website named after it now, UVB-76. The V was actually a Z but that is a whole other story! It was also known as the Buzzer. Not to be confused with the Russian Woodpecker, which had another unrelated purpose. As of 2019, this station was still operating under new callsign ANVF.
Lincolnshire Poacher ran by British Intelligence was another interesting one. Yosemite Sam turned out to be from the United States, New Mexico to be specific although it was claimed to be a "test". Here you heard the cartoon voice before the numbers.
Again as mentioned already, in the US there was the Cuban Five as they are referred to used radio to send spy transmissions to Fidel after "keeping their eye on" refugees.
There are many resources on the web dealing with these topics where you can read up on this even more. If you have some additional sources, please drop me a line! I would love to get this list updated as I do have an interest in checking up on these once in a while. Enjoy!
If you find something incorrect, have an update, or if there is something you would like to see added to the site, drop me a line here. I like to keep things as up-to-date and as accurate as possible. Thank you! - Lucas / W6AER