Since World War I, countries around the world have been broadcasting mysterious numerical messages via shortwave radio. Though concrete evidence is hard to come by, the general consensus is that these coded messages are meant for undercover agents operating abroad. And one particular Russian station may have an even more sinister purpose. Featuring computer engineer Andrus Aaslaid, historian Maris Goldmanis, and documentary photographer Lewis Bush.

Follow Dallas on Instagram, TikTok and LinkedIn.Watch our video shorts on YouTube, and join the discussion on Reddit and Facebook.Become a monthly contributor at 20k.org/donate.If you know what this week's mystery sound is, tell us at mystery.20k.org.We’re hiring! Visit defactosound.com/careers for more info.To dive deeper into numbers stations, visit numbers-stations.com and priyom.org, or check out Lewis’ book, Shadows of the State.Episode transcript, music, and credits can be found here: https://www.20k.org/episodes/thebuzzerVisit monday.com/podcast for your free two-week trial.Go to everyplate.com and enter the promo code, 20k199 to get started for just $1.99 a meal.Learn more about your ad choices. Visit megaphone.fm/adchoices