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.
