Quilotoa is a water-filled crater lake and the most western volcano in the Ecuadorian Andes. The 3-kilometer (2 mi)-wide caldera (cauldron-like hollow space) was formed by the collapse of this dacite volcano following a catastrophic VEI-6 ( Volcanic Explosivity Index) activity about 800 years ago. The caldera has since accumulated a 250-meter-deep (820 ft) crater lake, which has a greenish color as a result of dissolved minerals. Fumaroles are found on the lake floor and hot springs occur on the eastern flank of the volcano.

The lake surface is located at (3,500 meters (11,483 ft). The total volume of water stored in Lake Quilotoa is (0.35 cubic kilometers (0.084 cu mi). According to local inhabitants, the lake level has been slowly declining over the last 10 years. The village of Quilotoa and the associated crater is also a popular destination within the 'Quilotoa Loop' and is a common starting point for the 'Quilotoa Traverse', a multi-day village to village hiking route.

More Info: en.wikipedia.org