The human voice can only carry so far, but a whistle can travel for miles. In the mountains and ravines of La Gomera, in the Canary Islands, a language of whistles has been used for centuries to transmit long distance messages with amazing accuracy.
The messages can travel so reliably because the whistling is not a simple signaling system, like smoke signals, where there’s a limited set of meanings that have been agreed upon beforehand, but a full language, namely, Spanish. The language, known as Silbo Gomero, involves transposing the sounds of Spanish into whistles.
However, it takes a lot of practice to hear the whistles as Spanish. Here’s a demonstration of Silbo Gomero: