A two-front problem
Venice's relationship with water is famously complex, defined by a dual geological and climatic predicament: the city is slowly sinking as the sea around it is simultaneously rising. The sinking, known as subsidence, occurs at a rate of 1-2 millimeters per year. This process is primarily natural, caused by the compaction of the soft sediment layers beneath the islands. The city's foundations rest on sand, silt, and clay deposited over millennia, which gradually compress under the weight of the buildings.
Tectonic movement also contributes. The Adriatic plate, on which Venice sits, is slowly moving beneath the Apennine Mountains, causing the entire region to tilt and drop slightly in elevation. This geological process contributes to a slight eastward tilt of about one to two millimeters per year.
Compounding the subsidence is the global rise in sea levels, a result of climate change. In the Venetian lagoon, the sea is rising at a rate that has recently accelerated. Between 1993 and 2019, the average sea-level rise was measured at approximately 2.76 mm per year. This is an increase from the 1.23 mm per year average recorded between 1872 and 2019. The combination of the sinking land and rising water results in a relative sea-level rise that makes flooding events, known locally as "acqua alta," more frequent and severe.
An industrial legacy and modern solutions
While the current subsidence is mostly natural, human activity dramatically accelerated the process in the mid-20th century. Industrial facilities in the nearby Marghera district pumped massive quantities of groundwater from aquifers deep beneath the lagoon. This removal of water reduced pressure in the underground sand layers, causing the ground above to compact and sink rapidly. Between 1950 and 1970, this activity caused Venice to sink by an average of 10 cm, with some areas of Marghera dropping by 14 cm. Recognizing the danger, authorities halted the groundwater extraction in the 1970s, which significantly slowed the rate of subsidence.
To measure these subtle but persistent movements, scientists use advanced geodetic techniques. Satellite-based Interferometric Synthetic Aperture Radar (InSAR) can detect millimeter-scale changes in ground elevation across the entire city. This technology works by comparing radar images of the same area taken at different times to create a map of ground displacement.
To protect the city from the worsening floods, Italy constructed a massive system of mobile floodgates called the MOSE (Modulo Sperimentale Elettromeccanico) project. Operational since 2020, MOSE consists of 78 retractable gates positioned at the three inlets to the Venetian Lagoon. During normal conditions, the hollow gates are filled with water and rest on the seabed. When a tide forecast exceeds 110-130 cm, compressed air forces the water out, causing the gates to pivot upwards and temporarily seal the lagoon from the Adriatic Sea. The system is designed to protect Venice from tides up to 3 meters high.