This language is based only on whistles
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:
It doesn't sound much like Spanish — or any other spoken language, for that matter — but close studies of the whistling have revealed that there are fine acoustic distinctions in it that recreate, in an indirect way, the acoustics of speech.
How does it work?
We are accustomed to hearing whistled pitch changes as intonation or melody, but in Silbo Gomero, they indicate vowels and consonants. Notice what happens to your tongue position when you alternate between the vowels "ee" and "oo" while trying to hold your lips still. It moves forward and up for "ee," and down and back for "oo." Now do that alternation while whistling. Your mouth is basically a slide whistle! Vowels are positions on the slide.
In Silbo Gomero the highest pitched vowel is "i" and the lowest is "o" with the other vowels somewhere in between. At the French site Le Monde Siffle (The World Whistles), there is a cool test you can take to train yourself on the difference. Try it.
Consonants are more complicated. They aren't particular tones, but movements to or from vowel tones. For example, for sounds normally produced with the tongue at the ridge behind the teeth ("coronal consonants") like t, d, r, n, and l, the transition to a following vowel falls sharply from a very high starting point while for consonants with other places of articulation like p, b, f, and g it falls from a lower starting point.
Consonants are further distinguished by features like whether there's a smooth transition or a brief interruption, relative loudness, steadiness, or rate of decay of the sound. The phonetics of whistled language are covered in depth in this paper by Annie Rialland. There are a number of dynamic aspects of whistling that can be exploited for fine distinctions. You just have to learn how to hear them and use them.
Who speaks it?
In 2009 UNESCO added Silbo Gomero to its wonderfully-named list Masterpieces of the Oral and Intangible Heritage of Humanity. Schoolchildren in La Gomera now learn the fine distinctions of whistling in school. Use of the language had been on a sharp decline since the 1950s, but recent preservation efforts have led to a revival. This documentary from Time, The Last Speakers of the Lost Whistling Language, shows modern Silbo in action.
Is it the only one of its kind?
While Silbo Gomero is certainly unusual, it is not the only whistling language. There are whistling language in Turkey, Greece, Mexico, and France, which you can learn more about at Le Monde Siffle. Because whistling languages are just a technique for transposing a spoken language, theoretically, any language can be represented this way. It may be the case that Silbo was originally used before the Spanish conquest of the Canary Islands, when the inhabitants spoke a Berber language, and it was later adapted to Spanish. No reason it couldn't be adapted to English too. Who knows. These "oral and intangible" systems are never written down. Maybe long, long ago some shepherds in England had their own whistling way of sending messages great distances across the hilly moors. It would be a skill worth developing, for those moments when you just can't get good cell phone reception.