Silbo Gomero is a unique form of communication through whistling used for thousands of years in La Gomera. It reproduces Castilian Spanish –the language of its inhabitants– in a series of different whistles, and has been passed down for centuries from fathers to sons as simply another tool for working in the countryside . Today, and since the government made it a school subject in 1999, it is the only whistled language in the world. It is calculated that around 22,000 people can speak it correctly . This is why when you're walking in the mountains of La Gomera, listen closely –you may still be able to hear the whistles of two people having a conversation.