Blue Lake ( or Plavo jezero) is a karst lake located near Imotski in southern Croatia. Like the nearby Red Lake, it lies in a deep sinkhole possibly formed by the collapse of an enormous cave. The total depth from the upper rim is around , while water depth varies with season. In spring, when the snow from surrounding mountains melts, it can reach , and in 1914 it reached , overflowing the southern rim. The lake is a popular destination for hiking and sight-seeing.
Maximum dimensions of the lake are around , but they significantly vary due to big changes in the water level. At the end of the summer the lake may completely disappear.
In 1907 a road was built descending to the lake. In 1942, an earthquake caused a large rockfall, resulting in reduction of the lake's depth.