Exsurgence de Port-Miou, Underwater karst spring in Cassis, France
Exsurgence de Port-Miou is an underwater spring along the coast near Cassis where water pushes up from deep limestone channels beneath the seafloor. The upwelling creates a visible disturbance in the surface water as fresh and saltwater meet at the opening.
Exploration of the system began in the 1950s when researchers started mapping the underwater passages. Subsequent dives expanded knowledge of how far these flooded tunnels extended into the limestone.
The spring is a place where visitors can observe how nature shapes the landscape through water. Swimmers and divers who pass by often stop to watch this natural phenomenon at work.
The site is easily reached from Cassis by walking along the coastal path where you can observe the spring from above the water. To see more of the underwater system, snorkeling or diving on calm days works best.
This is not just a single opening but the visible exit of an enormous flooded cave system hidden beneath the landscape. The hidden network reveals how water has sculpted the limestone bedrock over thousands of years.
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