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.

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