Silbo Gomero is an old whistling language used on the Canary island of Gomera. It is based on a regional variety of spoken Spanish with a small number of vowels and consonants. But the language existed before the Spanish arrived - it has been adapted to Spanish since.
The island of Gomera contains mountains, valleys and ravines, making it an arduous process to go from one place to another, and the whistling language came about to help inhabitants communicate with one another. It can be heard up to two miles away.
Locals say that the whistling language was in widespread use in the 1940s and 50s. Like slang and secret languages it was often used by locals to ensure that outsiders couldn't understand them. It later declined as economic difficulties caused some locals to emigrate, and as the road network improved, and these days mobile phones enable long-distance communication.
There was a revival of interest in Silbo Gomero in the 1990s, and it is now a compulsory subject in primary schools on the island.
The BBC website has an article on Silbo Gomero, with a video from a school classroom and a radio clip. See it here.