Pozzo del Merro is a flooded

sinkhole A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are locally also known as ''vrtače'' and shakeholes, and to openi ...

in the countryside northeast of

Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption ...

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical ...

. Situated at the bottom of an 80 m conical pit, at 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 without reaching the bottom. The second ROV, "Hyball 300", reached without touching down either. A third dive in 2002 with the more advanced "Prometeo" robot reached the bottom at , but discovered a narrow passage continuing horizontally. The sinkhole, similar to the Zacatón cenote, was formed by volcanic activity eroding the

carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolosto ...

References

Sinkholes of Italy Metropolitan City of Rome Capital {{geology-stub