Scientists have warned for decades that Shanghai, one of the world’s largest and most densely populated coastal megacities, has been slowly sinking due to the way humans use the land beneath it. This problem is not new. According to reports, historical records show that parts of Shanghai have dropped by more than 2 meters over the past century due to over-pumping of groundwater and soft, compressible urban soil. Subsidence makes the city more likely to experience flooding, rising sea levels and damage to infrastructure.But even as it all sinks, Shanghai isn’t suddenly falling apart or disappearing. Engineers and officials have found ways to slow the rate at which the ground is sinking. They have begun taking steps such as reducing groundwater pumping, extracting water into deeper aquifers, and replenishing aquifers with treated surface water rather than allowing the land to dry out as water is pumped. These actions act like invisible supports beneath the city, arresting and slowing subsidence.Parts of Shanghai are still sinking, but the average subsidence rate has dropped significantly since its peak in the mid-20th century. Understanding how and why this happens requires digging deep into the underground, porous sediments and human-managed water systems that together determine the risks and responses in this megacity.
What causes cities like Shanghai to sink?
Shanghai sits on the flat, soft sediments of the Yangtze River Delta. Sedimented over thousands of years, these sediments contain layers of silt, clay and sand that behave like a sponge when loading or unloading water. When these tiny pore spaces fill with water, they help support the weight of the buildings, roads and soil above them. However, when too much fluid is withdrawn, the supporting pore pressure decreases and the sediment compacts under its own weight. This process causes land subsidence, or surface subsidence.In the early and mid-20th century, rapid industrial development and population expansion led to Shanghai pumping out large amounts of groundwater. People extract water for industry, agriculture and growing urban needs. This, combined with the weight of high-rise buildings, resulted in land subsidence rates reaching or exceeding 10-15 centimeters per year in some areas in the late 1950s and early 1960s.The subsidence phenomenon is not unique to Shanghai. Other cities, including Mexico City and Long Beach, California, have experienced similar land subsidence as fluids are pumped from below the surface. In each case, the behavior of fluids in underground sediments plays a central role in whether the ground sinks, rises, or remains stable.
Invisible support: How water injection works
If removing water causes sediment compaction, scientists and engineers ask a simple question: What would happen if we put some liquid back in? Cities began trying to inject water into depleted aquifers and old oil or gas fields, rather than treating the water table as drainage. Injecting water under pressure increases pore fluid pressure in the sediment, providing additional support and reducing compaction rates.In Long Beach, California, a water injection program started in the late 1950s helped reduce land subsidence in the area from as much as 9 meters to much lower levels. Engineers use treated seawater and formation water injected through hundreds of wells to slow the compaction of depleted layers. The scheme is widely cited as one of the first large-scale applications of liquid injection to control subsidence.Shanghai takes a related but slightly different approach. Authorities have gradually reduced over-pumping of groundwater, diverted water to deeper strata, and installed recharge wells to inject treated river water underground. As a result, the once-rapid rate of subsidence has slowed to about one centimeter per year in recent decades. That doesn’t mean cities are rising again, but it does mean the rate of decline has slowed significantly.
Why reducing settlement is important
For a coastal city like Shanghai, even a few centimeters of ground movement can have a big impact. Land subsidence, along with rising sea levels due to climate change, makes flooding more likely, damaging infrastructure such as subways and roads, and raising flood prevention costs. In real life, every centimeter less settlement gives planners and engineers more time to improve drainage, strengthen levees, or rethink how infrastructure is built.But experts caution that fluid injections and artificial replenishment do not cure the disease. Much of the compression that occurs before these practices begin is almost permanent. Scientific studies of subsidence in places like Mexico City show that even if groundwater is allowed to rise, the ground rarely returns to its original height.Risks associated with fluid injection also include the potential to reactivate faults or trigger small seismic events if water is added too quickly or inappropriately. As a result, modern projects rely on detailed monitoring systems, including GPS, satellite radar and borehole instruments, to track minute changes in surface and underground pressures.
The future of Shanghai and other sinking cities
Many low-lying megacities in China and elsewhere face similar challenges. Historical records show that since the early 20th century, central Shanghai has sunk by more than two meters due to the dual effects of groundwater extraction and urban development.Government agencies and researchers are now carefully monitoring ground movements and integrating lessons from other cities to manage subsidence. Techniques such as artificial recharge and careful groundwater management are becoming part of wider urban planning strategies aimed at reducing long-term risks.While Shanghai has not yet completely sunk, its experience and that of other sinking cities shows that invisible processes beneath our feet can shape the fate of entire metropolises. As sea levels continue to rise and cities continue to grow in size, understanding and managing these processes remains a key priority.
