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.
Great Blue Hole
This site was made famous by Jacques Cousteau, who declared it one of the top five scuba diving sites in the world. In 1971 he brought his ship, the "Calypso", to the hole to chart its depths. Investigations by this expedition confirmed the hole's origin as typical karst limestone formations, formed before rises in sea level in at least four stages, leaving ledges at depths of , , and . Stalactites were retrieved from submerged caves, confirming their previous formation above sea level. Some of these stalactites were also off-vertical by 5˚ in a consistent orientation, indicating that there had also been some past geological shift and tilting of the underlying plateau, followed by a long period in the current plane. The tilt indicates that this was a movement of the land, rather than a rise in sea level alone.