As a geologist and cave diver I have always been very charmed by flooded caves. These places, like secret doors in the heart of the earth, are evidence of the silent work of water erosion on the calcareous rocks during the geological ages. 

Sometimes an amazing, unexplored place is waiting just a few miles outside of a big city. 

This is the story of the discovery, exploration and study of one of the world's deepest flooded sinkhole: the "Pozzo del Merro" (Merro Well).

The "Pozzo del Merro" is a sinkhole in the Cornicolani Mounts located a few kilometers East of Rome (Latium Region, Central Italy). It is like a big funnel in the limestone of Lias age (about 200 millions years). The name "Merro" is a Central Italy dialectical word meaning a "very deep and steep precipice". The mouth of the sink has a circular boundary that is 492 feet diameter. The dry section, with walls covered by a luxurious vegetation, is deep about 230 feet deep. On the bottom lies a small circular lake 100 feet in diameter, hiding a dark liquid abyss more than 1016 feet deep.
Incredibly, until my team took on the challenge, no one had ever before dove this sinkhole, and it is only 45 minutes outside of Rome! 

I do not know why it had never been explored. Perhaps it was because the lake in the bottom appears to be a very shallow pool filled with some water. Carrying heavy scuba gear down the lake is a hard job and few people want to do this for nothing. The only people usually willing to take the risk are geologists and explorers!

In the early part of 1999 the study of the "Merro" became the main argument for my degree thesis in Hydrogeology at the Geology Department of the University of Rome. A cave divers team was formed to explore the flooded sinkhole. 

The team included Simone Formica, Diver Instructor, Riccardo Malatesta, Member of the Scuba Team of the Italian Fire Brigade, and me, geologist and scientific diver.
We started diving to collect data along with water and rock samples. In addition we wanted to map the conduit and monitor the main chemical parameters. 



Diving in an environment like the "Merro" requires cave diving and deep diving techniques. The first problem was to carry the scuba gears down to the lake surface. Fortunately along the walls were some rocky steps which were built by the Regional Water Society of Rome in the 1970 experimental purposes. However the last 50 feet required us to use ropes and tackles to reach the water.
Entering the water was like penetrating a world that was silent, foreign and dark. 

A greenish glow surrounded us due to the presence of floating plants on the water's surface. Going deeper, the beams of the scuba lamps were the only lights in this water-filled abyss. During the descent the only link with the surface was the line, which was spooled out of the reel. Along the white limestone walls of the conduit were many secondary caves, like black eyes looking at the divers. These openings are just blind alleys to nowhere. The only way to proceed in the exploration was to go down, further and further in the main passage. Eventually the conduit became tight but there was enough space for a full equipped cave diver to swim through. Although the water was clear the presence of silt along the walls made visibility a bit difficult. In this case correct use of the line was the only way to exit the cave.

Trimix:
Our standard scuba gear included: twin tanks with manifold, double regulators (first and second stage) with pressure gauge, reels, dry suits, BCDs with double air bags and helmets with two primary lights and four safety lights. The use of such redundant lights was due to the scientific probing of the cave. In this case a light failure may have caused the loss of very important data. The wide-angle lightning, thanks to the two primary lamps, was helpful for the mapping and geological survey of the environment. Naturally this standard configuration changed with the different purposes of the dives. In my opinion, due to some years of scientific cave diving, there is not just one configuration style; however there is the right configuration that fits one kind of dive. In an hard environment like the "Merro", one must pay attention to optimize the gear used, because a second chance may not be possible.

For the shallow dives we used air and for the deeper Trimix. As scientist Trimix diving was the best solution to avoid nitrogen narcosis and to have a very clear mind. This is very important in scientific tasks where the precision is a must. We dived up to 328 feet on Trimix taking samples of water, mapping the main conduit and exploring the sinkhole. In these technical dives we used, as usual, stage bottles for the deco gasses. For easier swimming in the tighter sections of the karst conduit, the stage bottles were clipped along a strong nylon line that was fixed on the limestone walls from 230 feet to the surface. We used the line also as reference in the mapping work.

For deeper dives our choice was the use of some automatic submergible machines: the ROV (Remote, Operated, Vehicles). The ROV are machines with electric thrusters and video cameras. They are connected with the surface by a cable that is also used for the power supply and the data transmission. 

The use of ROV was possible thanks to the help of the Scuba Team of the Italian Fire Brigade, which owns and operates these devices for recovery purposes. The Firemen cleared overgrown vegetation from trails located in the dry section of the sinkhole. They then built a boogie to carry all the equipment down to the water's surface and constructed a floating device to hold the remote control station of the ROV.

In early 2000 we used two different ROVs to explore the sinkhole. The first of these, the "Mercurio (Mercury)" dived to the depth 689 feet (maximum operative pressure limit) without reaching the bottom. The second ROV "Hyball 300" reached 1016 feet without touching down either! 

"...Today the "Pozzo del Merro" is the deepest flooded sinkhole in the world that has ever been explored by ROVs..."

The spread karst erosion in the dry and submerged sections of the sinkhole showed evidence of the strong activity of chemical aggressive fluids raising up from faults. These are the same faults that favored the development of the "Merro". Chemical analyses of the water show a mineralization higher than the usual one in fresh spring water. This is because of the presence, in the neighboring area, of an ancient volcano: "The Albani Hills". Although the last volcanic activity ended tens of thousands of years ago the ancient fire is still burning in the depth of the ground, and the typical volcanic fluids are still rising through the crevices of the limestone around the "Merro" sinkhole.
Thanks to the Italian Firemen who made this study and subsequent article possible through their support, hard work and valiant efforts.