Located in Calanque de Morgeiou in Marseille, its entrance is located 37 meters underwater and was only ever known to public until 1991, when three divers got lost in the cave and died. The cave can now be accessed by divers through a tunnel.
Believe it or not, the cave was once inhabited as it was not always underwater. In fact, 150 prehistoric paintings and carvings on the walls are found inside. The current underwater location of the cave is due to the rise of the ocean during Paleolithic times.