The lake-filled Quilotoa caldera ?

The Quilotoa caldera was formed by the collapse of the Quilotoa volcano.
Following the 1280 Plinian eruption.
The term caldera was introduced by the German geologist Leopold von Buch.
Quilotoa is the most western volcano in the Ecuadorian Andes, Cotopaxi Province, Ecuador.

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(Crédits photo: Gabriel Castillo)