Kelimutu crater lakes in Flores Island, Indonesia
The westernmost lake is called the Lake of Old People and is usually blue, and the other two are called the Lake of Young Men and Maidens and Bewitched or Enchanted Lake, which are typically green and red, respectively. The colors vary on a periodic basis and have become a popular tourist destination.
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Hike map: http://ut.no/tur/til-trolltunga