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
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).
"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.
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
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
Thanks to the Italian Firemen who made this study and
subsequent article possible through their support, hard
work and valiant efforts.