The "GR20" is a famous hiking trail thru the mountains of Corsica.
(GR stands for "Grande Randonnée")
Last week, weather suddenly changed to cold and fog and 3 hikers died from hypothermia, on different parts of the trail.
Weather changes rapidly in the mountains. Do NOT go for a hike without an appropriate set of clothes in your pack.
URLs  (in French ...): 
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News article in LE Monde * 
the trail