Venice is an architectural marvel built upon a sprawling archipelago of 118 small islands within the shallow Venetian Lagoon. This body of water is not the open sea, but a sophisticated tidal system of salt marshes, mudflats, and channels fed by the Adriatic Sea through three main inlets. The question is not merely how the city was built, but why its founders chose such an unstable, waterlogged foundation. The answers lie in a complex interplay of historical necessity, ingenious engineering, and modern environmental challenges that now threaten its existence.
Why the Lagoon Was Chosen
The decision to settle in the lagoon was driven by the need for security and refuge during violent upheaval on the Italian mainland. As the Western Roman Empire collapsed in the 5th century AD, the people of Venetia faced relentless invasions from Germanic tribes, including the Huns and Lombards. The marshy, difficult-to-access islands of the lagoon offered a natural, impenetrable barrier against these land-based armies.
The shallow waters and shifting mudflats acted as a sophisticated defense mechanism, making it nearly impossible for foreign forces to launch an effective attack. Settlers, initially on islands like Torcello, traded dry land for safety, leveraging the lagoon’s natural topology as their fortress. This strategic placement not only ensured their survival but also positioned the nascent community to eventually become a maritime power, controlling trade routes between Western Europe and the East.
How Venice Was Built on Water
To transform the unstable silt and sand of the lagoon into a stable platform, the early Venetians developed an enduring engineering solution. They began by driving millions of wooden piles deep into the soft, muddy subsoil until the tips reached a much firmer layer of compressed clay and sediment known as caranto. This dense layer provided the necessary resistance to support the incredible weight of stone buildings constructed above.
The piles themselves were made from dense, water-resistant woods such as oak, larch, and alder, often sourced from forests on the mainland and Istria. Crucially, the wood remains permanently submerged in the oxygen-deprived (anaerobic) mud and saltwater of the lagoon. This lack of oxygen prevents the aerobic bacteria and fungi that cause decay from thriving, effectively preserving the wood indefinitely.
Over centuries, the mineral-rich saltwater has slowly permeated the wood fibers, a process that partially petrifies and hardens the piles into a stone-like material. Above this dense “upside-down forest,” a wooden platform was laid, topped with a non-porous foundation layer. This final layer was constructed using Istrian stone, a dense limestone that resists saltwater and forms the base for the city’s iconic structures.
Factors Driving Increased Water Levels
Despite the brilliant original engineering, Venice now faces severe challenges from natural, human-caused, and global environmental factors driving increased water levels. One primary factor is subsidence, the sinking of the ground upon which the city rests. Part of this sinking is a natural geological process of the underlying sediments compacting, estimated to cause the city to settle by about 1 millimeter per year.
Historically, the rate of subsidence was dramatically accelerated by industrial activity on the mainland, which involved extracting vast quantities of groundwater from beneath the lagoon during the mid-20th century. Although this practice was banned decades ago, its lasting effect has permanently lowered the ground level of Venice, making the city significantly more vulnerable to flooding. The sinking ground means the relative sea level is rising faster in Venice than in many other coastal areas.
This local vulnerability is compounded by eustatic sea level rise, which is the global increase in the average height of the world’s oceans due to climate change. As global temperatures rise, melting ice sheets and the thermal expansion of seawater directly contribute to a higher Adriatic Sea level. Measurements indicate that the average sea level in Venice has been rising at a rate of approximately 2.76 millimeters per year since the 1990s, a rate higher than the historical trend.
These long-term factors are what make the acqua alta, or high water phenomenon, a recurring crisis. Acqua alta occurs when high astronomical tides coincide with specific meteorological conditions, particularly strong, sustained southerly winds known as the Sirocco. The Sirocco wind pushes water from the Adriatic Sea into the Venetian Lagoon, creating a significant storm surge. When this surge combines with the already rising relative sea level, it results in the extreme flooding events that increasingly inundate the city’s lower areas, like St. Mark’s Square. The MOSE barrier system, a series of movable floodgates, was constructed to mitigate the effects of these frequent and severe high water events.