Pozzo del Merro (English: the Well of 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 deepest underwater vertical cave in the world. In 2000 two ROVs (Remotely operated underwater vehicle) 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. 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.
The sinkhole was formed by volcanic activity eroding the carbonate rock.