Jezioro Turkusowe, Artificial lake in Konin, Poland.

Jezioro Turkusowe is an artificial lake on the border of Konin and the Kazimierz Biskupi commune, formed in a former brown coal open pit. Its greenish-blue water fills the excavation, and a flat strip of land with reeds and grasses borders the far shore, backed by a dense line of tall trees.

The lake formed in the 1970s after the closure of the Gosławice brown coal mine, when water from the nearby power plant filled the excavation. The site later became a public destination.

The name Jezioro Turkusowe comes from the greenish-blue colour the water took on after power-plant waste was discharged into the former Gosławice lignite pit and its minerals dissolved. The colour is the result of that alkaline chemistry, not of any tropical setting, and bathing is banned because of it.

The lake lies on the border of Konin and the Kazimierz Biskupi commune in Wielkopolska. Bathing is forbidden because the water is strongly alkaline. The shore is a flat strip of reeds and grasses backed by a wall of tall trees, so the water is best reached on foot along that bank.

The turquoise tint is not natural: water drained from the power plant carried waste whose minerals turned the lake alkaline and gave it its colour. Because of that strongly alkaline reaction, bathing in the water is forbidden.