The existence of a city below sea level suggests a constant, tenuous negotiation with nature. “Below sea level” means the ground elevation is lower than the local Mean Sea Level (MSL), which is the calculated average of high and low tides. If the protective infrastructure were removed, the area would naturally flood until the water level equalized with the surrounding sea. The persistence of human habitation in these basins is a testament to sophisticated engineering that actively defies the natural tendency of water to seek the lowest point.
Geological and Man-Made Factors That Lower Elevation
The initial formation of low-lying city areas is often a result of large-scale geological forces or deliberate human action. Tectonic forces, for instance, can create natural depressions known as sedimentary basins, where the Earth’s crust is stretched or pushed down. These basins accumulate thick layers of sediment over geological time and naturally settle below the level of the sea.
Another powerful geological influence is the aftermath of the last Ice Age, known as Glacial Isostatic Adjustment. Massive ice sheets pressed down on the land, causing the Earth’s mantle to bulge outward around the edges, forming an area called a forebulge. Now that the ice has melted, the land that was directly under the ice is slowly rising, while the forebulge areas, like the US East Coast, are sinking as the Earth’s crust relaxes.
In contrast to these natural processes, some low elevations are entirely man-made, most famously the polders of the Netherlands. Polders are tracts of land reclaimed from the sea, lakes, or marshes by constructing dikes and then draining the enclosed area. This intentional drainage allowed for the cultivation of fertile land and urban expansion.
Essential Infrastructure for Water Defense
The survival of a city below sea level depends on a comprehensive, multi-layered system of engineered barriers and drainage mechanisms. The first line of defense is the containment structure, typically massive earthen embankments known as dikes or levees. These barriers protect the land from surrounding bodies of water, preventing flooding from the sea, rivers, or lakes.
Behind these barriers, an intricate network of canals, reservoirs, and specialized pumping stations manages the internal water balance. Low-lying areas lack natural drainage, meaning that all rainwater, seepage, and wastewater must be actively removed. This is often achieved through a ring canal system, which collects the water and channels it to a pumping station.
Pumping stations are constantly operational, lifting the accumulated water from the low-lying basin and discharging it over the dike or levee into a higher-elevation body of water, such as a river or the sea. While water is sometimes released by gravity through sluice gates during low tide, modern electric and diesel pumps ensure continuous discharge against infiltration and rainfall.
The Role of Ground Subsidence
Beyond the initial low elevation, many cities face the threat of ground subsidence, a dynamic process where the land surface actively sinks over time. This ongoing lowering of elevation is frequently linked to human intervention in the subsurface. Over-extraction of groundwater is a primary global cause, particularly in cities built on layers of unconsolidated clay and silt.
When water is pumped out of underground aquifers faster than it can be replenished, the resulting loss of fluid pressure causes the aquifer sediments to compact permanently, which lowers the ground surface. A similar process occurs with the extraction of hydrocarbons. Removing oil and gas from subsurface reservoirs reduces the pressure supporting the overlying rock layers, leading to reservoir compaction.
Subsidence is also accelerated by the natural compaction and oxidation of soft sediments like peat, especially when water is drained for development. This process creates a condition of relative sea level rise, where the combined effect of the land sinking and the global sea level rising significantly increases the flood risk for the community.
Cities That Exist Below Sea Level
Rotterdam, Netherlands, which sits approximately 90% below sea level, embodies the triumph of the polder system. The city is protected by the massive Delta Works and relies on pumps and a multi-layered water management strategy to keep its land dry and its large port operational.
New Orleans, Louisiana, presents a different case, with much of the metropolitan area sitting below the level of the Mississippi River and Lake Pontchartrain. The city’s low elevation is primarily due to the natural compression of deltaic sediments and the subsidence caused by draining marshland for urban expansion. Its defense relies on extensive levees and a powerful network of pumps capable of removing millions of gallons of rainfall per minute.
Venice, Italy, faces a threat compounded by both natural and human-induced sinking, including historic groundwater extraction and tectonic plate shifts. To combat increasingly frequent high tides, the city recently activated the MOSE project. This system uses 78 massive, mobile steel barriers that can be raised at the three inlets of the Venetian Lagoon, separating the lagoon from the Adriatic Sea during storm surges.