1988, Noel P. James, Philip W. Choquette, editors,Paleokarst, Springer Science & Business Media,→ISBN,page36:
Fontaine de Vaucluse, France, is the deepest knownkarst spring, -315 m. Lower parts were explored by a robot submersible. It is probably the ascending limb of a single loop in a syncline.
karst spring on Wikipedia.Wikipedia