Engineering Rome

The Layers of Underground Rome

By Astrid Bowden

All photos taken by author unless otherwise stated

1. Introduction

While walking around Rome, it is easy to look up at the hundreds of years of history ingrained in everyday buildings – but what about what lies beneath? For a city with thousands of years of such dense history, there is a lot to be learned from the many hidden artifacts buried within the soil as well as the various ancient dwellings that are now completely encased underground. In an attempt to encompass all of the ways that ancient Romans have repurposed and developed their underground space I have visited and analyzed three main locations- the Colosseum, the San Clemente Basilica and the Hills of Monteverde. While ancient Romans had more than an acute interest in creating their own subterranean wonders, such as the San Clemente Basilica, many locations and artifacts were left behind completely on accident. Over a few millennia the roman people were afflicted time and time again with floods, earthquakes, fires, war and – on a more positive note – their own technological advancement. With each of these occasions it was inevitable that a few artifacts here or there would be taken away by a flood or buildings would be nearly destroyed. Remarkably, it did not scare them away and they proved to repurpose these spaces time and time again. Ancient Romans did not create a lot of waste in their construction and would instead repurpose or build on top of damaged buildings. This not only creates a strong sense of history but also means that a large amount of significant space is hidden underground, some of it almost entirely forgotten.

This combination of destruction, restoration and random chance has created layers of roman history, some that will never be discovered. I am fascinated with this idea of the unknown and in this paper I will work to explain the different wonders of underground Rome and their impact on our understanding of Roman history.

Figure 1. Three Layers of Roman History

2. Background

2.1 Flooding of the Tiber

The Tiber River is an ancient one with great significance to Rome since the cities very first creation. The river was a very convenient source of fresh water which fed Roman citizens, without it such prosperous agriculture would not be possible. Equally as important, it was an important route for trade and travel as the river fed right into the Mediterranean Sea. However, the Tiber River also wielded great power and could destroy or damage much of the city with a single flood – and it happened often. There are some surviving records, suggesting that there were at least 33 floods between 424 BC and 398 BC which is an 812-year period (Aldrete 14). The water level rise could be so dramatic that the Romans could get around by boat for somewhere between 3-7 days after the flood. This frequent and dramatic flooding deposited a lot of alluvial sediment and could raise the elevation of the ground by at least a few inches. This soil, while being highly fertile, does not have great construction properties as it is generally soft due to varying levels of silt and clay. Likely, this soil would be cleared from the main roads and houses and possibly left behind on grassy or agriculture areas due to its fertilizing properties. Over time, this would lead to significant elevation change especially around the edges of the river.

Depending on their construction method or location, some structures were more susceptible to damage during these violent floods. The most vulnerable were low elevation dwellings belonging to less wealthy romans. These were often constructed of sun-dried or mud brick and could expand and crumble when in prolonged contact with water, essentially melting. While it was cheap to skip the firing process, it is an essential step in ensuring the integrity of the brick. The firing process not only removes all the water from the clay but also binds the clay particles together in a chemical process that occurs during high temperatures (Pérez-Monserrat). This makes it more resistant to water damage and essential for creating durable buildings. If the buildings were not destroyed, the sediment filled water would have swept up any belongings and deposited or buried them somewhere far from their owners. A flood does not have to have considerable depth to cause great damage: a modern car can be easily washed away by floods of less than two feet in height. Additionally, possessions from neighbors could be mixed and would be indistinguishable from another in the unpredictable nature of a flood. In fact, there were legal texts to guide Romans in how to address lost and claimed property in a flood (Aldrete 99).

Figure 2. Artifacts Found in the Soil Beneath Rome on Display in the Metro

On the other hand, temples, basilicas, theatres and other significant Roman structures has a better chance of withstanding the flooding. These would be constructed of extremely thick walls of kiln-dried brick, concrete, rubble or stone that could not be harmed by water. Some were even erected on top of massive concrete foundations, giving them extra safety from the raging Tiber floods, and some can still be standing today. However, not all were immune and during an inundation in 1902, the Roman Forum partially flooded and was destroyed – the site of many important urban rituals (Aldrete 31). The aftermath of the flood would require significant cleanup. The Tiber would have deposited many inches of thick, wet mud on all roads and even the sides of buildings. This would have had to been cleaned off or simply built over. In the case of destroyed or very damaged dwellings, their owners may have elected to fill them with debris and build over rather than attempting repair. Today, a walk over any of the bridges in Rome reveals not only the elevation change but also the many drainage channels which exist today, in response to the once drastic flooding of the Tiber River.

Figure 3. Ponte Sisto Bridge over Tiber River

2.2 Great Fire of Rome and Rebuild

On the night of July 19th, AD 64 a fire began in the various merchant shops in Rome’s chariot stadium, Circus Maximus. This fire blazed on for over a week and consumed most of the city, devastating Rome. The emperor at the time, Nero, claimed that the newfound radical group of Christians burnt it down. However, most historians blame the emperor himself as he had schemes to rebuild Rome in his own image – which he did. While there were many wealthier neighborhoods made of stone and rubble that survived the fires, the common insulae constructed of sun-dried brick and wood burned quickly to the ground (Barrett). This level of destruction raised the ground level of Rome by several meters in one fiery event and laid a new foundation for the Rome that we know today.

It is a bit of an understatement to say that there was much to be rebuilt. Between cleanup, reconstruction and handling the healthcare of the injured Romans – it was like building an entire city from the ground up. Before any construction could begin, the debris first had to be cleared. It was used to fill partially burned buildings as well as the malaria filled marshes nearby. Nero had strict visions for the reconstruction, first on the list was to build himself a palace: the Domus Aurea.

Figure 4. The Domus Aurea (link)

This marked the first use of concrete as a building material in fine architecture. Beyond that, he directed the plan for the reconstruction of Rome. He restricted the height of buildings, requiring porticos, and choice building materials. However, instead of building them all himself he instead offered grants to those who wished to undertake such a task and so his strict requirements were not always followed. Nonetheless, most of Rome was rebuilt under his reign, despite his failing reputation and inevitable death four years later.

3. Underground Feats of Rome

3.1 Underbelly of the Colosseum

One of the most well-known architectural and engineering feats of Rome, the colosseum has a few of its own secrets. Over 14 stories tall, it was originally built to hold over 80,000 spectators to watch gladiatorial battles. To keep the games interesting additional scenery, fighters, and animals would appear on stage in a whim. This was all made possible due to the underground hypogeum: a maze-like system built to deliver any deadly wonder to stage in a matter of seconds. The hypogeum contained vast amounts of engineering including devices that not only held tremendous weight, but also guided cages, lifted and lowered platforms, all summing up to a very powerful elevator system that kept the games of the Colosseum lively. This was all done in a very cramped space and men were working in extremely hot conditions. Their efforts were coordinated by various signals such as organs, horns and drums so that a complex series of tasks could be accomplished in a short amount of time (Mueller). Over time, the Hypogeum became more and more disorganized after hundreds of years of repairs and modifications, and the disarray of walls can be seen from the ground floor.

Figure 5. Hypogeum

While the many rooms of the hypogeum serve a purpose for storing gladiators and animals, they also serve as the support for the area above. Like many roman buildings at the time, the walls are constructed with an outer layer of brick and filled with concrete. This concrete is made of cement and any aggregate that the romans could get their hands on including broken pottery, shards of brick and rocks. While modern concrete has more specific requirements for the aggregate and therefore higher strength all around, the sheer size and number of walls in the hypogeum supply more than enough strength.

Figure 6. Another View of the Hypogeum

Sometime in the 6th century, the last of the gladiatorial games were held in the colosseum after being abolished several times by different emperors during the rise of Christianity. Afterwards, the Colosseum lived many different lives, becoming the home for cobblers, blacksmiths, botanists and priests (Mueller). Meanwhile it was also slowly being buried under layers of dirt and rubble, vegetable gardens, and the storage of hay.  This incredible reuse of space that has occurred in the colosseum would have normally led to degradation of the structure and inevitable destruction. However, the colosseums incredibly robust foundation has kept it standing strong.

3.3 San Clemente Basilica

The San Clemente Basilica contains multiple centuries of reuse and renovation in one single building. Having approximately three stages of construction. The first and now deepest level was originally a mansion owned by Titus Flavius Clemens in the second century AD and was used for Christian worship as it was forbidden at the time. This level was thought to have been destroy in the Great Fire of 64 AD and was filled in with debris in order to build a new level directly on top of it (Nucelli). A century later, a two-story temple was built on the same site which would later be damaged and buried under the streets of the roman city after it was sacked by the Normans (Joan). Finally, a new church was built and finished in 1108 which still stands today, 60 feet higher than the original foundation at street level.

Diagram, engineering drawing

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Figure 7. Levels of San Clemente Basilica (link)

This basilica is one of the best examples of the elevation increase and Rome and how Ancient Romans adapted to this change by continuing to fill and build over old structures.

3.4 Caves of Monteverde

            Monteverde, a hillside neighborhood not far from the center of Rome harbors secrets just beneath its foundation. A vast network of underground quarries was formally discovered in October of 2013 by Roma Sotterranea but was known by a few as it resided just behind a set of doors in a car garage. It was discovered while archeologists were in search of the lost Jewish catacombs of Monteverde. Speleologists first entered by drilling a 23m shaft in the tufa and descending through ropes, later discovering an opening in an existing garage allowing for repeat visits and thorough  mapping of the entire system.

Figure 8. Map of the Monteverde Quarries

This map is a section of the quarry overlain by the map of the above-ground streets and buildings. It is a great way to visualize how large these quarries are. It is known that the halls are up to 90 meters in length, 30-40 meters wide and about 7 meters high making up an area of approximately 150,000 square meters, all beginning around 20m below ground surface (Lanzini). These are not natural caves, and instead are the product of extensive quarrying which took place up until the early 1900s by explosives. Since then, the caves have been used for many different functions throughout time including air raid shelters during the last world war. This is evidenced by the existence of defensive walls immediately at the entrance to protect people in the caves from the shock of the bombs. The residents of each time period also left many artifacts mostly comprised of rudimentary ceramic dishes and silverware.

Figure 9. Pile of Miscellaneous Artifacts in the Monteverde Caves

While some of these artifacts were simply left behind, the quarries were also used extensively for disposal of waste. Piles of previously infected dishware from a hospital can be found as well as mounds of construction waste and aggregate.

These caves have also caused structural concerns for the buildings constructed immediately above them. In order to reduce the risk of collapse, previous engineers have installed columns within the caves in order to reinforce the ground above. Looking back at figure 444, little dots can be found all over the map. These are the various columns that have been constructed. As a result, many different types of columns constructed throughout time are found scattered throughout the quarry, some made of brick and others made completely of concrete. This human intervention is a great example of how underground Rome can be discovered and then completely forgotten in just a 100-year timeline.

Figure 10. Concrete Columns in the Quarry
Figure 11. Brick and Concrete Columns in the Quarry

Since the original excavation and subsequent years of use, the quarries have largely been forgotten. This has allowed nature to reclaim the area, in a way. Many stalactites can be seen forming as ground water drips from the ceiling of the quarry in some places. Also, natural lakes have formed in special areas. All in all, this quarry has been used in many ways throughout time and holds many of its own historical stories, each important to getting closer to the entire story of underground Rome.

Figure 12. Stalactites Forming in the Quarry

4. Conclusion

From the quarries in Monteverde to the Hypogeum of the colosseum, it is evident that Rome keeps some of its oldest secrets underground. It is no wonder so many historical sites ended up underground between the endless inundations of the Tiber river and the Great Fire of 64 AD: ancient romans learned how to adapt to their surroundings and subsequent elevation increase by building up. It is important that the engineering feats of underground Rome be properly accounted for so they are not forgotten, such as in the case of the Monteverde Caves. This can be done thanks to organizations like Roma Sotterranea and other archaeologist in Rome. This is not only for conservation, but safety as well. When components of underground Rome are forgotten sink holes can occur and the structural integrity of buildings above ground can be compromised. Throughout time, underground Rome will continue to develop and it is important that engineers, speleologists, and archeologists collaborate to prevent disasters and preserve history all at once.


Aldrete, Gregory S. “Floods of the Tiber in Ancient Rome”. Johns Hopkins University Press, 2006.

Barrett, Anthony. “Rome Is Burning Nero and the Fire That Ended a Dynasty”. Princeton University Press, 2020.

Dembskey, Evan J. “Aqueducts of Ancient Rome.” Feb. 2009, pp. 1-195.

Joan E. Barclay Lloyd. “The Building History of the Medieval Church of S. Clemente in Rome.”

Journal of the Society of Architectural Historians, vol. 45, no. 3, 1986, pp. 197–223. JSTOR, Accessed 18 Sep. 2022.

Lanzini, M., et al. “The Recent Discovery of the Underground Quarries in Monteverde (Rome)”, 2015

Mueller, Tom. “Secrets of the Colosseum.”, Smithsonian Institution, 1 Jan. 2011,

Nuccilli, Antonio. “Basilica San Clemente.” Basilica, 2022,

Pérez-Monserrat, Elena et al. “Roman brick production technologies in Padua”. Journal of Cultural Heritage, 2022,

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