Pozzo del Merro

Useful Information

Location:	Via Cavour, 00010 Sant'Angelo Romano RM. Near Mentana, northeast of Rome. A1 exit Guidonia-Montecelio, turn right on SP28b to Guidonia, left to Pichini, at the motorway right SP23a towards Palombara 5 km, left towards Mentana 2.2 km, right Loc. Selva. (42.0393124, 12.6805073)
Open:	no restrictions. [2024]
Fee:	free. [2024]
Classification:	Doline
Light:	n/a.
Dimension:	VR=392 m, D=35 m.
Guided tours:	self guided
Photography:	allowed
Accessibility:	no
Bibliography:	Roberto Palozzi et. al. (2010): The underwater exploration of the Merro sinkhole and the associated diving physiological and psychological effects, International Journal of the Society for Underwater Technology, Vol 29, No 3, pp 1–10, 2010. pdf Valerio Baiocchi et. al. (2018): First geomatic restitution of the sinkhole known as ‘Pozzo del Merro’ (Italy), with the integration and comparison of ‘classic’ and innovative geomatic techniques Environmental Earth Sciences. Valume 77. DOI pdf G. Caramanna (2002): Exploring one of the world’s deepest sinkholes: the Pozzo del Merro (Italy), Underwater Speleology, February 2002 (pp.4-8). G. Caramanna (2005): Scientific Diving and ROV Techniques Applied to the Geomorphological and Hydrogeological Study of the World’s Deepest Karst Sinkhole, (Pozzo del Merro – Latium – Italy), Proceedings of the American Academy of Underwater Sciences Symposium, 2005, University of Connecticut at Avery Point, Groton. pdf
Address:	Riserva Macchia di Gattaceca e Macchia del Barco, Via Tiburtina, 691, 00159 Roma, Tel: +39-06-67663301, Fax: +39-06-43562126. E-mail:
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then. Please check rates and details directly with the companies in question if you need more recent info.

History

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".

Description

Pozzo del Merro (Merro Well) is one of the deepest collapse dolines in the world. Located in a plain, the doline with its steep walls is water-filled to a level about 80 m below ground. The water-filled shaft below is explored up to a depth of 392 m, which results in 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 this 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, and the current depth of 392 m is 322 m below sea level. In other words, the cave below drained to a much lower sea level, at least 350 m below today. The obvious explanation is the Geology Messinian Salinity Crisis, when the strait of Gibraltar was closed and as a result the Mediterranean dried out almost completely.

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 diving 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.