Pozzo del Merro

Pozzo del Merro is a flooded sinkhole in the countryside northeast of Rome, Italy. Situated at the bottom of an 80 m conical pit, at it is the second deepest underwater vertical cave in the world. In 2000 two ROVs were sent to explore its depths; the first, the "Mercurio " reached its maximum operative depth of without reaching the bottom. The second ROV, "Hyball 300", reached without touching down either. A third dive in 2002 with the more advanced "Prometeo" robot reached the bottom at, but discovered a narrow passage continuing horizontally.
The sinkhole, similar to the Zacatón cenote, was formed by volcanic activity eroding the carbonate rock.
The sinkhole is part of the natural reserve Macchia di Gattaceca e Macchia del Barco.