Blue Hole (Big Pine Key)
The Blue Hole is an attraction on the island of Big Pine Key in the Florida Keys. It is an abandoned rock quarry that was used for nearby road fills and Henry Flagler's Overseas Railroad. The water it contains is mostly fresh and is used by various wildlife in the area, such as birds, snakes, alligators, key deer and green iguanas. It is part of the National Key Deer Refuge.
Blue Lake (Croatia)
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 220 meters, while water depth varies with season. In spring, when the snow from surrounding mountains melts, it can reach 90 m, and in 1914 it reached 114 m, overflowing the southern rim. The lake is a popular destination for hiking and sight-seeing.