Blue Hole (Castalia)
The Blue Hole is a fresh water pond located in Castalia, Erie County, Ohio, in the United States. From the 1920s to 1990 the Blue Hole was a tourist site, attracting 165,000 visitors annually at the height of its popularity, partly because of its location on State Route 269, about southwest of the Cedar Point amusement park in Sandusky, Ohio.
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.