On La Gomera in the Canary Islands, the locals devised an unusual way of communicating across the deep valleys that carve up the island - they whistled. This whistling was not merely a way of announcing one's presence, it was a fully fledged language. One that was sophisticated enough to be translated into the language of the Spanish conquerors when they arrived in the 16th century. (The YouTube video at the top of this post demonstrates an example of this, with English subtitles.) Since then, this unique language has been declining and was on the verge of dying out when it became a part of the syllabus in La Gomeran schools in 1999. Since then, there has been something of a revival of the language and in 2003, La Gomera played host to the First International Congress of Whistled Languages. (via Neatorama)
(There are other examples of whistled languages in Turkey, the French Pyrenees, Mexico, Alaska, Nepal, China, and Greece.)