Lake Canterno is of Carsic origin and lies in the heart of the Ernici Mountains. It touches the towns of Ferentino. Fiuggi, Fumone and Trivigliano. The original lake filled up when a sinkhole, through which a stream flowed into the ground, was obstructed by debris, the increasing water pressure then caused the hole to reopen and the lake emptied once again. In 1997 the Canterno Lake Regional Nature Reserve was established.