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 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.^[2]

The sinkhole, similar to the Zacatón cenote, was formed by volcanic activity eroding the carbonate rock.^[3]

References

1. https://web.infinito.it/utenti/s/simonant/merro-english.htm
2. https://web.infinito.it/utenti/s/simonant/merro392.htm
3. https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62638.htm