Pozzo del Diavolo, or Devil's Well, is the only known volcanic cave in Italy's Lazio region. It is located above Vico Lake in the beautiful beech forest of Monte Venere. (The forest is part of the UNESCO's Primeval Beech Forests of Europe transnational network of protected sites.) At 507 meters (1,663 feet) above sea level, Lake Vico is the highest volcanic lake in Italy and the beech forest of Monte Venere is among the lowest in the country (most beech forests are located above 900 meters (2,900 feet). 

Pozzo del Diavolo is about 10 meters  (32 feet) deep and 40 meters (130 feet) long. The entrance is about five meters (16 feet) wide. The sloping floor of the cave is littered with volcanic rocks that presumably collapsed from the entrance and from the ceiling. The lake filled up a caldera that formed almost 140,000 years ago, after intense volcanic activity shaped the landscape of the Cimini Mountains. Monte Venere was originally an island, before the Etruscans drained the lake to create new agricultural land.

Remains of pottery from the 5th and 4th centuries B.C. were found inside the cave and are currently exhibited at the local museums in Rome (Pigorini) and Valentano.

According to a very ancient legend, the cave is the result of Hercules' fury: The legendary hero smashed his club in the earth, forming the cave, from which water sprang and formed the Lake of Vico. This scene is represented in a fresco by Federico Zuccari in nearby Palazzo Farnese in Caprarola with Alessandro Farnese depicting himself as a new fresco for his work on the local aqueduct.

It is unclear, however, what the devil has to do with all this; like many other places in Italy, it seems like anything that could not be explained would be simply the devil's work!