THE EMISSARY OF LAKE NEMI AND THE SHIPS OF CALIGULA
by Carlo Cestra
Fig.01 Emissary entrance in Roman times.
Before the excursion inside the emissary of Lake Nemi, I had already dealt with the subject having made a digital reconstruction on behalf of the Archaeological Superintendence of Lazio. This work is part of a documentary lasting a total of 15 minutes I made for the Museum of Roman Ships of Nemi which is also visible at the National Roman Museum at Palazzo Massimo in Rome. The digital reconstruction that I realized focuses on recovery efforts of the two ships of Lake Nemi made over the centuries.
Fig.02 In 1446 the first attempt of the Nemi's ships recovery was performed by Leon Battista Alberti, a Renaissance genius, who designed a special wooden bell for diving. The bell was equipped with a small opening with a glass from which he could look out. Practically the first rudimentary diving bell of history.
Fig.02b The Leon Battista Alberti's bell was lowered from a specially built raft.
The existence of the two ships on the bottom of the lake was known since a long time thanks to the fishermen that, with their nets, sometimes recovered random fragments of wood and other objects.
Fig.03 The wreck of one of the two ships as it probably appeared, emerging from the muddy bottom of the lake. Due to the slope of the seabed, the two wrecks were located at a depth that ranged from 5 to 15/20 meters from bow to stern. The wooden hull has survived thanks to the mud that has preserved it for 20 centuries.
These findings, over the centuries, aroused some people curiosity so that they tried to recover the ships.
Fig.04 The first attempts to retrieve the ships were built by hooking, with ropes, the protruding parts of the ships. Obviously the only result was to strip away some fragment of wood causing serious damages to the old structures.
Entering the emissary I thought I know much more than I actually knew. And this is a reason for me to address the issue again, in greater depth, and produce a detailed digital reconstruction and faithful, both historically and archaeologically.
The first surprise was discovering that the current entrance to the tunnel is not actually the original entrance made for leveling the lake. The current entrance was made in Roman times and was most likely modified and adapted also to enable the launching of the two ships of Caligula. At the entrance in fact there are vertical grooves along which ran the wooden gates that were lowered to close the water inlet. In this way, the lake level could rise up to reach the ships that, once finished on the shore near the present museum, could float and thus be launched.
The original entrance, however is much older and is located about 12 meters above the current one. At that time, in fact, the lake level was much higher.
With the digital reconstruction which I created, I have tried to highlight the entrance structure, its shape and its functionality.
Fig.05 Cross section of the complex structure of the emissary entrance with the stone filters and a shutter, which is visible on the far right, in the raised position.
Immediately after the entrance there is a conical shaped chamber which is delimited by two big stone filters (currently partly collapsed). These filters prevented any large debris (for example, branches of trees) to penetrate inside and clog the duct. The debris remained locked inside this room and then were removed by maintenance workers.
Fig.06 Detail of a stone filter.
This structure was built in square work with tuff blocks. Once finished, it was closed with a barrel vault, leaving the appropriate openings for the inspection, access to maintenance teams and to allow the raising and lowering maneuvers of the gate valves.
Fig.07 The entrance seen from above. Note the conical shape of the accumulation chamber, where the debris coming from the outside remained trapped.
The original purpose of the effluent was to level the lake to prevent water from flooding the sanctuary of Diana, located on the opposite banks of the lake, subject to frequent swamping. The lake level in fact could oscillate because of water intake from natural underground channels.
Inside the emissary, shortly after the entrance, there is the original tunnel that had the opening 12 meters higher.
Note that there are only two “wells”, one at the beginning and one at the end of the emissary from which the two teams of diggers started to work. In this regard, I immediately thinked about how workers more than two thousand years ago had been able to accomplish such a work. A tunnel 1.6 km long dug by two teams of workers who met halfway in the heart of the mountain without the help of computers and GPS. The technique used was called "cultellatio": the profile of the hill above was propped up with poles, placed at a regular distance and height between them. In this they had a measuring unit and a reference to proceed in a straight line and with a certain slope inside the tunnel. It was also used the light emitted by a fireplace that further facilitated the rectilinear course of the excavation. A fire at the beginning of the tunnel can be seen from a distance and then used to drive straight ahead. Remarkable is the technique used for the collimation of the light: the tunnel walls are not perfectly straight but always slightly wavy, almost they appears like continuous corrections of the trend of the excavation. Actually these undulations were used to collimate the light of the hearth in the distance and always keep it in the center of the tunnel as they proceeded with the excavation.
Another big surprise was discovering how the emissary is not just a tunnel but a far more articulate work. Inside there are service tunnels, bypass and attempts to avoid basalt blocks which are harder than the lava rock (that makes up most of the route). In some sections, where it was necessary to proceed in the hard basalt, you can still see signs on the rock that suggest, for their extensive and circular shape, the use of rudimentary but effective machinery. In another part you can see a tunnel, probably out of order due to of a collapse. That has been used as a dumping ground for excavated material (easier to leave them there, than take them outside). In short, a work that has suffered several changes over the centuries and which has had different roles other than that for which it was originally designed and used. By the Romans who modified and used it for the launching of Caligula's ships until the '30s when the emissary was reused after a restore and consolidation work, to partially drain the lake and thus allow the final recovery of the two hige ships of Caligula. For this purpose, gigantic suction pumps, manufactured by the company Riva of Milan, were installed near the entrance, to empty the lake pouring the water into the emissary.
Fig.08 A concrete platform, on which four water pumps were installed, was built on the shore of the lake near the entrance of the emissary. Also a transformer room, to provide electrical energy to the pumps, was built.
Fig.08b The pumps poured water in a tub constructed outside the entrance. Then the water flowed into the duct and the emissaryt poured it into the Ariccia valley, on the other side of the hill.
The challenge was unbelievable for its time and not without technical problems (including the collapse of the structure of the pumps due to a landslide of muddy ground). But after many months of pumping, the lake level dropped about 20 meters allowing the recovery of the two ships.
Fig.09 After five months of pumping the first ship appeared.
The men who built the emissary would never have imagined that after two thousand years their work would still be fully functional and used for a different purpose.
The ships had incredible size. They were about 70 meters long and over 20 meters wide. Two huge barges that had to carry a palace and a temple made of marble. The construction technique of the hulls was called "bearing shell": the shell plating was made first; then it was reinforced with ribs placed inside and fixed with nails. In order to support the weight of the overlying building and temple (made of brick, with paneling and marble statues, and decorations in gold, copper and bronze) over the bridge was laid a layer of terracotta pillars that were used to distribute the weight and cushion the inevitable movements and settlements of wood.
Fig.10 Cushioning and weight distribution system. The palace was built on top of this "sandwich structure" in which terracotta pillars were placed.
The two ships had probably a great religious value and were built by Caligula on the example of similar sacred ships built by the pharaohs in Egypt and dedicated to the cult of Isis.
The two boats even if structurally almost identical had very different features. The temple-ship could move on the surface of the lake and maneuver thanks to the oarsmen who were placed in special structures protruding from the hull. This vessel was probably used to carry out religious ceremonies in honor of the goddess Diana. The palace-ship had to be towed because she had no oarsmen that would have disturbed the tranquillity enjoyed by the guests on board, including the emperor.
Fig.11 The structure of the hull of the temple-ship with the protrusions where the rowers were placed.
The finding of the Nemi's ships was one of the most important archaeological discoveries, as it has allowed, thanks to the perfect state of preservation of the hulls, to understand and examine in depth the shipbuilding techniques of the ancient Romans as well as to become aware of their technology. It were found, in fact, part of the on-board equipment including winches, pumps and taps.
Fig.12 Cross section of the stern of the ship where the hand-operated bilge pumps (which were used to empty the bottom of the hull from the water that inevitably infiltrated inside) were probably located. The pump has been rebuilt according to the few clues found on the lake bottom.
The two ships were completely destroyed by fire during the Second World War.
My interest for the digital reconstruction of the these ships dates back to around 2001 when I created the first 3D models of the hulls using the drawings and reliefs that I found in the book "The ships of Nemi", written by Guido Ucelli, the engineer who was in charge of the recovery operations.
Fig.13 Reconstruction of two ships of Caligula in the middle of Nemi' lake.
I also created a virtual digital tour to explore the archeological excavation as it was when the lake was dried out and the two ships were found.