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After almost dying out in the 1970s, a kind of whistling language is now undergoing a revival.
Silbo Gomero, as it's known, has become a compulsory subject in one school in Spain's Canary Islands.
The whistling approximates the sounds of the Spanish language.
It can be heard from two miles away, and was used to communicate across valleys and ravines before the arrival of telephones.
Vocabulary 詞匯:
dying out 正在消失的
undergoing a revival 趨於重振
compulsory 必需的，強迫的
approximates 模仿，接近於
ravines 峽谷