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 392 m 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 (Mercury)" reached its maximum operative depth of 210 m without reaching the bottom. The second ROV, "Hyball 300", reached 310 m without touching down either. A third dive in 2002 with the more advanced "Prometeo" robot reached the bottom at 392 m, but discovered a narrow passage continuing horizontally.

The sinkhole, similar to the Zacatón cenote, was formed by volcanic activity eroding the carbonate rock.