Kelimutu Monyet ("Mount Boiling Monkey")

Kelimutu is a small, but well-known, Indonesian compound volcano in central Flores Island with three summit crater lakes of varying colors. The traditional name of Kelimutu translates locally as Keli=Mount and Mutu=Boiling, corresponding to the fact that the volcanic mountain is always active. The western lake, Tiwi Ata Mbupu (Lake of Old People) is commonly blue. Tiwu Nua Muri Kooh Tai (Lake of Young Men and Maidens) and Tiwu Ata Polo (Bewitched, or Enchanted Lake), which share a common crater wall, are commonly colored green and red, respectively, although lake colors periodically vary. The scenic lakes are a popular tourist destination and have been the source of minor phreatic eruptions in historical time.

The science of the Kelimutu lakes is relatively well-known, and the changes in color and temperature are concluded to be related to volcanic activity and the resulting fluid flux at vents at the bottom of the lakes. The lake colors periodically change due to adjustments in the oxidation-reduction status of the fluid of each lake, and also considering the abundance of different major elements, such as iron and manganese. Oxidation-reduction status depends on the balance of volcanic gas input and rainfall rate and is thought to be mediated by the groundwater system in the volcano itself.

The colors in the lakes change independently from each other, as each has its own unique connectivity to the underlying volcano's activity. Between January and November 2016, the colors of the craters changed six times. Although it is widely believed that the changes are unpredictable, it is more accurate to say that the lack of any regular monitoring of the volcanic system precludes scientists from having the data necessary to drive widely available predictive models.