Blue Hole (Red Sea)
The Blue Hole is a diving location on the southeast Sinai, a few kilometres north of Dahab, Egypt on the coast of the Red Sea.
Castalian Spring
In Castalia, Ohio, named after the famed site, the Blue Hole limestone configurations were tourist attractions for many years and are now fish hatcheries. Waters in the blue holes are of a constant temperature and never freeze over despite northern Ohio's cold temperatures. The water which averages 30,000 gallons an hour was first used to supply a local mill.