Sorgente del Gorgazzo
The Gorgazzo Spring is a natural cavity created by the erosion of water favoured by a tectonic discontinuity in the form of a fault running right through the stretch where the Gorgazzo spring is located.
The Gorgazzo is basically a deep trap that has not yet been fully explored. The small lake that can be seen on the surface was formed by a vault collapsing under the pressure of water, whose origin is uncertain.
More (The link you are accessing might not be in English.)