In 2010, as the Colombian government was preparing to rescue 16 soldiers held by armed FARC guerrillas, it looked in vain for a way to alert the soldiers without tipping off their captors. Finally Colonel Jose Espejo arranged to have local radio stations broadcast a pop song that contained a message in Morse code, which the soldiers had learned in basic training but that the guerrillas likely wouldn’t recognize.

The lyrics run, “In the middle of the night / Thinking about what I love the most / I feel the need to sing … About how much I miss them.” And hidden at three points in the song (1:30, 2:30, 3:30), in Morse code, is the message “19 people rescued. You are next. Don’t lose hope.”

“The hostages were listening to our own stations, so we made sure the song was played,” Espejo told The Verge. “The code message said, ‘you’re next’ because the hostages thought if they ran away, they would die in the jungle. We let them know that our troops were nearby.”

It worked. “We know of hostages who heard the message,” Espejo said, “and were able to escape and provide information that led to the release of more hostages.”