Pozzo del Merro

Useful Information

Location: Near Mentana, northeast of Rome.
Open: no restrictions [2007]
Fee: free [2007]
Classification: KarstDoline
Light: n/a.
Dimension: VR=392 m, D=35 m.
Guided tours:
Address: Riserva Macchia di Gattaceca e Macchia del Barco, Via Tiburtina, 691, 00159 Roma, Tel: +39-06-67663301, Fax: +39-06-43562126. E-mail: contact
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then.
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1999 first exploration of the pozzo by divers.
2000 a depth of 310 m reached with a Remote Operated Vehicle (ROV).
2002 a depth of 392 m is reached by the ROV "Prometeo".


Pozzo del Merro (Merro Well) is one of the deepest collapse dolines in the world. Located in a plain, the dolines with its steep walls is waterfilled to a level about 80 m below ground. The waterfilled shaft below is explored up to a depth of 392 m. This makes a total depth of 472 m. The walls are covered by a thick vegetation, a path with rock stairs leads down to the lake.

Beneath those statistical data, the location of this shaft is quite strange. The plain around the doline is 150 m asl. The lake is 80 m below, at 70 m asl. As a matter of fact the current depth of 392 m is 322 m below sea level.

The well was explored during the last years by a team of geologists and cave divers around Dr. Giorgio Caramanna. In 1999 he studied the Merro for his thesis in Hydrogeology at the Geology Department of the University of Rome. After some dives with Trimix to a depth of 100 m the maximum depth for humans was reached, but no bottom found. Subsequently the exploration continued with small remote controlled submarines called Remote Operated Vehicle (ROV). Various attempts increased the depth to 392 m, still without finding the bottom.

The Pozzo del Merro is located in the Riserva Macchia di Gattaceca e Macchia del Barco. The area between the Tiber valley and the Cornicolani mountains is protected for the botanic value of its forest fragments and for the intense karst phenomena. There are grotte (caves), sventatori (swallow holes), and pozzi (dolines). The underground is composed of Mesozoic limestones, about 200Ma old from the Lias, the lower Jurassic. The limestones are covered by Plio-Pleistocene sands, clays of sea origin, and tufa produced by the activity of the Sabatino Volcano during the Quaternary.