Only 20min from Marseille, are in the commune Cassis, the rivers Port Miou and Bestouan. These exceptional submarine rivers in the Calanques National Park (see "Diving in the National Park of the Calanques") deserve special attention and are considered the two most beautiful of France.
Note: these are not places where everyone can venture even by being a very good diver with a lot of experience, this requires specific equipment and adapted training (cf article «The specificities of cave diving») !
These subterranean aqueducts, which are almost completely submerged, are natural treasures.
The Port Miou river leads into the calanque of the same name; The water outlet is visible from the road that overlooks the cliff. The natural entrance has two passages; We find a first exit in the open air about fifty meters from the entrance into a well through which passes a pipe containing the famous red mud that is poured into the canyon off Cassis (which we hope to stop next ). Port Miou has been recognized over 2km. Xavier Méniscus stopped at a depth of 224m in a horizontal section. Beyond that all remains to be discovered but the very high distance and depth make it very difficult to study the continuation of the river. It is noteworthy that at 500m from the entrance is an underground dam unique in the world with an access by an artificial well and that had been constructed without great results in order to stop the sea and to have a stock of Fresh water upstream. The infiltrations in the limestone massif being numerous the water remains brackish in the whole of the zone hitherto explored.
The Bestouan, less deep than his sister (33m maximum) has been recognized on more than 3km. Explorations have so far stopped on a geological obstacle that has formed collapses in both final galleries. Speleologists still hope to take a look at this river upstream and continue to dig to continue their research.
Indeed, questions persist on these networks, where do they come from? What massifs actually feed them? Is there a drinking water reserve in the area that could be useful to the population? Everything remains to be understood ...
To know more about these real underground rivers, see beautiful photos, videos, and follow the work of studies in progress, you can consult the site of the association cassis rivières mystérieuses (http://www.rivieresmysterieuses.org/).
If you are confirmed / certified cave divers only, these spots are for you:
Access (forbidden without boat, tolerated out of season): From the parking of the tennis club located on the peninsula. Launching in the bottom in the creek that you will have to cross in immersion because of the navigation (to provide a S80 in relay). Connect the output to the main line. The cavity is vast, we see shrimp at the entrance, the halocline is beautiful, the bell near the entrance is worth the detour and reminds the cenotes.
Getting there: From the parking lot of the Bestouan beach, the launch is made from the beach. Leave left hand when entering the harbor (watch out for the boats). The entrance equipped with a chain is at 1m of depth, under the cliff in the axis of a concrete forward. It is a passable site by Mistral but not when the swell comes from the sea. The first 45 meters are narrow and lead to a well emerging on a bell. The current is quite strong and the gallery is splendid. Watch out for ears!