The lake dates at least to the Pliocene and possibly is as old as the upper Miocene, an age of about five to perhaps ten million years. The hills hosting the lake are Cretaceous limestone (about eighty million years in age), a porous rock type. Deep below the waters of the lake are rocks of older formations that were once part of the super-continent of Pangaea (that is, up to about 270 million years ago). Because these basement rocks are impermeable, waters seeping through the limestone collect in the basin of Kastoria, and the lake is the surficial expression of this water table.
As this water percolates through these hills, it reacts to form a mild acid that dissolves this rock and forms caves and fissures as it passes. One cave, with an entrance on the lakefront itself, is called “The Dragon’s Cave.” Local myth placed a dragon inhabitant of this stalagtite-filled cavern as lording over piles of gold before being slain by the king, with the dragon’s body lost in the lake.
Today the waters of the lake are not more than ten meters deep. In the past, during the time that mammoths drank from its waters, its depth was as much as fifty meters, and covered the area where the city of Kastoria now stands.
Photo from the region’s official site:http://www.kastoria.gr/
Rassios, A., 2004. A Geologist’s Guide to West Macedonia, Greece. 120 pp.