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 (1,286 ft) 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 (690 ft) without reaching the bottom. The second ROV, "Hyball 300", reached 310 m (1,020 ft) without touching down either.^[1] A third dive in 2002 with the more advanced "Prometeo" robot reached the bottom at 392 m (1,286 ft), but discovered a narrow passage continuing horizontally.^[1]

Pozzo del Merro
Location	Sant'Angelo Romano, Lazio, Italy
Coordinates	42.039298°N 12.680456°E / 42.039298; 12.680456
Depth	c. 392 m

The sinkhole, similar to the Zacatón cenote, was formed by volcanic activity eroding the carbonate rock.^[2]

The sinkhole is part of the natural reserve Macchia di Gattaceca e Macchia del Barco (created in 1997).^[3]